Fix typos in num_failures_by_type for json results.
Fix typos in num_failures_by_type for json results.

This changes a few occurrences of "Failure" -> "FAIL" and "Pass" -> "PASS"
to conform to the doc format. The existing data is not actually being
used yet, but the flakiness dashboard needs the correct values.

Also fix a typo in run_mojo_python_tests that was preventing us from
running individual tests on the command line.

R=ojan@chromium.org, viettrungluu@chromium.org, grt@chromium.org

Review URL: https://codereview.chromium.org/429013004

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@286504 0039d316-1c4b-4281-b951-d872f2087c98

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