Add support for caching passing test results

[csamak]

This is not ready to merge now. Sharing as-is for feedback and help getting it ready to merge.

Some known issues:

• The session prefix for test output is removed.
• Console output does not indicate cache hits like it does for build actions, though the logs show cache queries/hits as expected.
• The absolute path is in the CAS, so pairing with something like --unstable-allow-all-tests-on-re for relative paths may be required.
• It needs a CLI flag.

See #183

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