run-tests: ignore ENOENT failures when removing old .err results When the same test runs in multiple threads and the previous run was a failure, the threads can race to delete the error output. This fixes that.

run-tests: stop storing start/stop times in a dict by test name This resolves the last breakage in run-tests that prevented me from running a single test many times in several threads in parallel. This will be useful for testing potential fixes to flaky tests.