Merge to XFA: Parallelize run_corpus_tests.py.

- Use the number of cores as the default -j value
- Fall back to old code for -j 1

TBR=nparker@chromium.org

Review URL: https://codereview.chromium.org/1398793003 .

(cherry picked from commit fd751f28cecce61ab36038799043639d570e0b26)

Review URL: https://codereview.chromium.org/1395253002 .
3 files changed