`runDailyTest()` calls `test.cleanup()` before returning, removing the entire checkout directory, along with coverage data, and the git repo.

This breaks both `postDailyResults()` and `postCoverageResults()`.

Instead use a callback to handle the daily results. This way we can keep the `defer` to clean up even in case of error.

Change-Id: I9730d7dd8ac9c3a19d82d07a68325afdf38bfd40
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44748Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>