Filter out system headers on macOS.

Fixes bazelbuild#14969.

RELNOTES:
Bazel now no longer includes system headers on macOS in coverage reports (bazelbuild#14969).

Closes bazelbuild#14971.

PiperOrigin-RevId: 433753228

Author: phst

tools/test/collect_coverage.sh

@@ -235,6 +235,7 @@ LCOV_MERGER_CMD="${LCOV_MERGER} --coverage_dir=${COVERAGE_DIR} \
   --filter_sources=/usr/bin/.+ \
   --filter_sources=/usr/lib/.+ \
   --filter_sources=/usr/include.+ \
+  --filter_sources=/Applications/.+ \
   --filter_sources=.*external/.+ \
   --source_file_manifest=${COVERAGE_MANIFEST}"