LLVM lit assumes control of the test parallelism when running a test
suite. This style of testing doesn't play nicely with build systems like
Buck or Bazel since it prefers finer grained actions on a per-test
level. In order for external build systems to control the test
parallelism, add an option to disable `.lit_test_times.txt` under the
`--skip-test-time-recording` flag, thus allowing other build systems
to determine the parallelism and avoid race conditions when writing
to that file. I went for `--skip-test-time-recording` instead of `--time-tests` in
order to preserve the original functionality of writing to `.lit_test_times.txt`
as the default behavior and only opt-in for those who do _not_ want
`.lit_test_times.txt` file.
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