Fix the coverage review findings: branches, stale reports, exit statusfeature/per-test-coverage

- gcov is run with --branch-probabilities; without it the JSON has no
  branches, so GCC runs had none in coverage.json and coverage.lcov.
- PrepareCoverageDirectory also removes coverage.json, coverage.lcov and
  coverage.profdata of an earlier run, so a run that writes no reports
  leaves none to be read as its own.
- WriteCoverageReports returns notes and failures apart. A profile the
  tools cannot read or a report that cannot be written counts as a missing
  report and fails a run whose tests passed; no profiles or missing tools
  stay notes. The paths are announced only for the reports written.
- ProfiledAttempt takes the last attempt whose profile exists, so a final
  retry or repeat that crashed before writing one no longer hides the
  coverage of the attempt before it.
- The per-test llvm-profdata/llvm-cov and gcov runs use as many threads
  as the run had workers; results keep the order of the tests.

Tests: earlier reports removed and no failure without profiles; an
attempt without a profile gives way to an earlier one; the fixture gains a
branch taken one way, checked in the combined LCOV for both LLVM and gcov.
Amalgamation regenerated.

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