fix: silence gcov's per-file summary under bazel coverage

[f0rmiga]

Problem

Bazel's collect_cc_coverage.sh invokes gcov without redirecting its stdout: the script declares gcov_log with a comment promising to save gcov's output there, but the variable is never used (still unfixed on Bazel master). gcov's per-file summary for the test's whole transitive C++ closure therefore lands in the test's test.log.

For large closures (e.g. pybind-heavy Python tests) this is megabytes of Lines executed:0.00% noise per test — one observed test.log was 9.2 MB with 237k gcov summary lines and 2 lines of actual test output. It buries the test's own output and can overflow --experimental_ui_max_stdouterr_bytes, at which point Bazel prints nothing at all for a failing test.

gcov itself has no quiet flag (-q is --use-hotness-colors), so the wrapper is the right interposition point.

Fix

Before the final exec, when the wrapped tool is gcov and COVERAGE_DIR is set (only true inside a Bazel coverage test action), redirect stdout to /dev/null. Bazel only consumes the .gcov.json.gz data files gcov writes to disk, never its stdout. stderr is untouched so real gcov errors still surface. All other tools wrapped by the same template are unaffected.

Verification

Ran bazel coverage on a downstream repo with --override_repository=gcc_toolchain=<this checkout>:

• gcov summary lines in test.log: 1877 → 0 (test.log 185 KB → 26 KB)
• combined lcov report byte-identical coverage content (same SF entries, same 392 nonzero DA records)
• a failing test now prints its log inline exactly like bazel test