Separated out tgc test results as a different status

[modular-magician]

This uses the statuses API to split out the results for terraform-google-conversion tests from generate-diffs. Note that this means that the script should only have a nonzero exit code if the script itself fails; the tests failing will no longer cause generate_diffs to fail (which is what we want. These test failures will be tracked by this new status instead.)

I can't test this fully because it requires rebuilding the docker containers. Doing tgc first lets us test it out with a relatively low-traffic component before doing the same conversion with e.g. tpg and tpgb.

I've tested this locally as much as I can - i.e. running the curl logic with a personal access token instead of the Magician's.

Derived from GoogleCloudPlatform/magic-modules#4865