Usability tweaks for code coverage
The new coverage tools introduced in #8949 (moved) have a few rough edges:
The cov-diff script wants two directories full of .gcov files, but the coverage script dumps them all in the repo root. There should be a way to specify a target directory to put them in.
If you modify the code, rebuild and re-run the test suite, the gcov instrumentation tries to merge the states into the existing gcda file and fails. There should be a makefile target to reset the coverage data by deleting just the gcda (not the gcno) files.