infra: individual test executables

[carsonRadtke]

We used have tests contained in a single executable. This was fine for testing, but it would be more convient to separate tests into indivudal modules so targeted changes could have targeted tests.

This change associates each test file with its own executable. We now have 14 tests for GSL each of which testing a different component.

Drive-by: Update the span tests to just check for C++14 and MSVC because that is easier than updating the MSVC version check each release.

[Copilot]

Pull Request Overview

This PR modernizes the GSL test infrastructure by separating tests from a single monolithic executable into individual test executables for each component. This change improves development workflow by enabling targeted testing of specific components.

• Splits the single gsl_tests executable into 14 individual test executables, one per test source file
• Simplifies MSVC version checks in span tests to focus on C++14 compatibility instead of specific compiler versions
• Maintains the same test coverage while improving modularity and development experience

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/CMakeLists.txt	Refactors build system to create individual executables for each test source file using a foreach loop
tests/span_tests.cpp	Simplifies MSVC version checks by removing specific version numbers and focusing on C++14 standard