Update unit testing guide with code coverage

src/content/docs/Products/SplashKit/Documentation/SplashKit Expansion/04-code-coverage.mdx

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+---
+title: Code Coverage
+---
+
+import { Steps, Tabs, TabItem } from "@astrojs/starlight/components";
+
+Code coverage provides a convenient way to see what parts of SplashKit are being tested. This is
+helpful for checking that existing tests are adequate, as well as for locating parts of the code
+would be good candidates for new tests. Consider generating a code coverage report after adding a
+new test, to see if there are any parts of the code that aren't being executed. This can sometimes
+help you come up with new test cases.
+
+## Pre-requisites
+
+To generate code coverage reports, install the required program based on your platform:
+
+<Tabs syncKey="platform">
+  <TabItem label="Linux/WSL">
+    Install lcov via your package manager e.g., `apt install lcov` (note that your Linux
+    distribution may use a different package manager rather than `apt`, and you may need to `sudo`
+    this command)
+  </TabItem>
+  <TabItem label="macOS">Install gcovr via homebrew: `brew install gcovr`</TabItem>
+  <TabItem label="Windows (MSYS)">
+    Code coverage is currently not supported on Windows with MSYS. Please use WSL if you're a
+    Windows user.
+  </TabItem>
+</Tabs>
+
+## Generating code coverage
+
+### From the command line
+
+<Tabs syncKey="platform">
+<TabItem label="Linux/WSL">
+<Steps>
+
+1. Change to the cmake project directory  
+   `cd ./projects/cmake`
+2. Select the coverage configuration preset  
+   `cmake --preset LinuxCoverage`
+3. Build the unit test executable  
+   `cmake --build build/ --target skunit_tests`
+4. Capture initial coverage  
+   `cmake --build build/ --target lcov-init-coverage`
+5. Run the test executable  
+   `../../bin/skunit_tests`
+6. Capture test coverage  
+   `cmake --build build/ --target lcov-capture-coverage`
+7. Generate a coverage report in HTML format  
+   `genhtml -o coverage-report/ build/lcov.info`
+8. Open `index.html` to view the report  
+   ![LCOV report](images/code-coverage/lcov-report.png)
+
+</Steps>
+</TabItem>
+<TabItem label="macOS">
+<Steps>
+
+1. Change to the cmake project directory  
+   `cd ./projects/cmake`
+2. Select the coverage configuration preset  
+   `cmake --preset macOSCoverage`
+3. Build the unit test executable  
+   `cmake --build build/ --target skunit_tests`
+4. Run the test executable  
+   `../../bin/skunit_tests`
+5. Capture test coverage and generate a report in HTML format  
+   `gcovr --root ../.. --filter ".*/coresdk/src/.*" --html-nested coverage-report/coverage.html`
+6. Open `coverage.html` to view the report  
+   ![Gcovr report](images/code-coverage/gcovr-report.png)
+
+</Steps>
+</TabItem>
+<TabItem label="Windows (MSYS)">
+
+Code coverage is currently not supported on Windows with MSYS. Please use WSL if you're a Windows
+user.
+
+</TabItem>
+</Tabs>
+
+### In VS Code
+
+Note that currently code coverage reports won't display properly with versions of the CMake Tools
+extension 1.21.36 and beyond. To work around this, you can install an older version of the extension
+by clicking the arrow next to Install/Uninstall (depending on if you have CMake Tools installed
+already) and selecting "Install Specific Version...". The latest version known to work is 1.20.53.
+
+<Tabs syncKey="platform">
+<TabItem label="Linux/WSL">
+<Steps>
+
+1. Go to the CMake Tools settings in VS Code, switch to the Workspace tab, and change the following
+   settings:  
+   **Coverage Info Files:**  
+   Click Add Item and add `${workspaceFolder}/projects/cmake/build/lcov.info`  
+   ![Coverage Info Files](images/code-coverage/option-coverage-info.png)  
+   **Pre Run Coverage Target:** `lcov-init-coverage`  
+   ![Lcov Pre Run Coverage Target](images/code-coverage/option-pre-run-lcov.png)  
+   **Post Run Coverage Target:** `lcov-capture-coverage`  
+   ![Lcov Post Run Coverage Target](images/code-coverage/option-post-run-lcov.png)
+
+2. Rebuild the test project, then run tests with coverage with this button  
+   ![Run Tests with Coverage](images/code-coverage/button-run-coverage.png)
+3. After the tests run, the test explorer pane in VS Code should update with the coverage info:  
+   ![Coverage Info](images/code-coverage/result-test-explorer.png)
+
+</Steps>
+</TabItem>
+<TabItem label="macOS">
+<Steps>
+
+1. Go to the CMake Tools settings in VS Code, switch to the Workspace tab, and change the following
+   settings:  
+   **Coverage Info Files:**  
+   Click Add Item and add `${workspaceFolder}/projects/cmake/build/lcov.info`  
+   ![Coverage Info Files](images/code-coverage/option-coverage-info.png)  
+   **Post Run Coverage Target:** `gcovr-capture-coverage`  
+   ![Gcovr Post Run Coverage Target](images/code-coverage/option-post-run-gcovr.png)
+
+2. Rebuild the test project, then run tests with coverage with this button  
+   ![Run Tests with Coverage](images/code-coverage/button-run-coverage.png)
+3. After the tests run, the test explorer pane in VS Code should update with the coverage info:
+   ![Coverage Info](images/code-coverage/result-test-explorer.png)
+
+</Steps>
+</TabItem>
+<TabItem label="Windows (MSYS)">
+
+Code coverage is currently not supported on Windows with MSYS. Please use WSL if you're a Windows
+user.
+
+</TabItem>
+</Tabs>