Restructure Nightly jobs to prevent flappers.

[brandonpage]

Other improvements:

• Fixed Code Coverage reporting.
• Use of two different Test Reporters (for now?):
  • One for PRs: This action understands and correctly reports "flaky" tests and tests that Firebase has re-run. But it does not display multiple test runs well (stating 2k+ tests ran and 1 failed does not tell me which API version it failed on). I have opened an issue asking for multi file support.
  • One for Nightlies: This action displays test results per file, so it is easy to tell what API levels tests pass or fail on. Reports tests that fail at first and succeed on the re-run as failures. I have commented on the issue asking for flaky/re-run support.
• Increased the number of re-run attempts for PRs to 2 and decreased it to 0 for nightly runs (largely due to reporters).