fix: Run all tests if changing test folders itself

[glasnt]

Description

Fixes b/309966756

Proof of concept: When a PR comes in that edits testing infra itself, then all tests should be run to verify the change.

This PR adds an escape hatch to return successfully from the "prune unchanged directories" step without pruning anything, in the event we are changing something to do with tests themselves.

Initial list of folders: build, test, .github (changes in any of these folders will cause all tests to run)

Additionally: adds the _RUN_ALL_TEST check (define as a substitution variable on Cloud Build triggers, for example) to escape the escape sequence, and don't even check about pruning, just run all the tests.

Note: given current infra, unsure if this change itself will allow tests to run.

Additional: In case of failures like #518 where git commands fail within the step, add set -e as to exit immediately on error (otherwise last exit code for step is used to confirm success of step). Resolves #518

Checklist

Readiness

• Yes, merge this PR after it is approved

[rogerthatdev]

The int test blocking this PR is failing on the sqlserver_instance_private_ip sample. I've added a skip to this sample so that we could troubleshoot it separately.

[msampathkumar]

/gcbrun

[iennae]

/gcbrun

[glasnt]

The updates to this PR show the limitations in the current CI limitation technique:

This PR is currently running the following tests:

Folders in scope for tests:
bigquery/
build/
cloud_sql/
test/

• It's not currently escaping early for build/, but
• It's also running the top level folders for all the tests for the merged content since this PR was created
• Sub-folder samples will currently trigger all tests within the same top level folder (as is blocking feat(cloudsql): Add mysql and postgres samples for enterprise_plus edition #509)
• If validation fails, tests will succeed after ~10 minutes without running any tests) (potentially failure case seen in feat(cloud_sql): Add cloud sql psc connectivity SQL server sample #560
• Any periodic tests from parent repo template won't run all tests (since this change overrides the functionality of the only int.cloudbuild.yaml config)

[glasnt]

Changes in #602 implemented some of these. Will re-create this PR after re-assessment.