This deprecates --vmodule (not available in absl, mapping it to --v for
a very minimal sort of compatibility) and adds the absl::log flags
--minloglevel, --stderrthreshold, --log_backtrace_at, and --log_prefix.
All gtest-based tests should now use add_gtest. Results from these tests
will be surfaced more prominently in GitHub Actions. Before this, it was
hard to find test failures in the log.