Stop swallowing System.err in JUnit tests

[jonahgraham]

test_printStackTrace overrides System.err with its own print stream to check functionality. But these tests did not restore System.err, so all System.err output for the rest of the test suite was lost into the last of these ByteArrayOutputStreams that was created.

The tests themselves have little intrinsic value as they really test nothing of SWT itself, but it does help achieve code coverage.

This was discovered when working on #2684 as I could not see the output when running all the tests.

[github-actions]

Test Results

  111 files   -  7    111 suites   - 7   14m 19s ⏱️ +19s
4 591 tests  - 56  4 575 ✅  - 54  15 💤  - 3  1 ❌ +1 
  274 runs   - 56    273 ✅  - 53   1 💤  - 3  0 ❌ ±0 

For more details on these failures, see this check.

Results for commit 785c322. ± Comparison against base commit c308fd1.

This pull request removes 56 tests.

AllWin32Tests ImageWin32Tests ‑ testDisposeDrawnImageBeforeRequestingTargetForOtherZoom
AllWin32Tests ImageWin32Tests ‑ testDrawImageAtDifferentZooms(boolean)[1] true
AllWin32Tests ImageWin32Tests ‑ testDrawImageAtDifferentZooms(boolean)[2] false
AllWin32Tests ImageWin32Tests ‑ testImageDataForDifferentFractionalZoomsShouldBeDifferent
AllWin32Tests ImageWin32Tests ‑ testImageShouldHaveDimesionAsPerZoomLevel
AllWin32Tests ImageWin32Tests ‑ testRetrieveImageDataAtDifferentZooms(boolean)[1] true
AllWin32Tests ImageWin32Tests ‑ testRetrieveImageDataAtDifferentZooms(boolean)[2] false
AllWin32Tests ImageWin32Tests ‑ test_getImageData_fromCopiedImage
AllWin32Tests ImageWin32Tests ‑ test_getImageData_fromImageForImageDataFromImage
AllWin32Tests TestTreeColumn ‑ test_ColumnOrder
…

♻️ This comment has been updated with latest results.

[jonahgraham]

FYI @SyntevoAlex I found out where System.err was going.

[mickaelistria]

FWIW, I've recently relied on AutoCloseable a lot for testing; it allows some good way of wrapping state relying less on Runnable.

For this case, it could go that way

class CaptureStderr implements AutoCloseable, Supplier<String> {
  ByteArrayOutputStream errContent
  CaptureStderr() {
     errContent = new ByteArrayOutputStream();
	 System.setErr(new PrintStream(errContent, true, StandardCharsets.UTF_8));
  }
  @Override
  public void close() {
    System.setErr(stderr);
    errContent.close();
  }
  @Override
  public String get() {
    return errContent.toString(StandardCharsets.UTF_8);
  }
}

and then tests can be written

try (new captureStdErr = new CaptureStdErr()) {
   // do stuff
  assertTrue(captureStdErr.get().contains("blah");
}