[9.3.0] Close the coverage post-processing FileOutErr before deleting its files

[fmeum]

Description

appendCoverageLog copies coverage.log and coverage.err into the test log and then deletes them, but the FileOutErr that the coverage post-processing spawn wrote them through was never closed. This closes it before the delete, and also closes it on the paths that rethrow, which previously leaked the handles.

Motivation

Deleting a file that still has an open handle is fine on POSIX, but not on Windows. Every affected test then fails with:

ERROR: ... Testing //tests/alwayslink_srcs:alwayslink_test failed: java.io.IOException: C:/b/skckkmg2/execroot/_main/bazel-out/x64_windows-fastbuild/testlogs/tests/alwayslink_srcs/alwayslink_test/coverage.err (Permission denied)

The bug is older than that, but it only became reachable for Bazel users with 8e68c24, which flipped --experimental_split_coverage_postprocessing to default true.

Build API Changes

No

Checklist

• I have added tests for the new use cases (if any).
• I have updated the documentation (if applicable).

Release Notes

Closes #30662.

PiperOrigin-RevId: 962724931
Change-Id: Icb52f15e33291676ee7f96a0d2d4b22235bd5536

(cherry picked from commit 53428ca)

9.3.0 adaptation: the release branch does not have master's if (testResultDataBuilder.getTestPassed()) wrapper around the coverage post-processing block and still has an extra writeOutFile(coverageOutErr.getErrorPath(), coverageOutErr.getOutputPath()) call before appendCoverageLog. The same four lines were applied at the equivalent positions, with coverageOutErr.close() placed before that first read of the coverage files.

Closes #30663