Run tests on GDK automatically on code changes

What does this MR do and why?

Run tests on GDK automatically in MRs with code changes. I intended to do this as part of !109286 (merged) but missed it.

The GDK E2E pipeline runs under almost the same conditions as package-and-test, except that package-and-test runs manually on code changes until we're able to handle the load.

We want the GDK E2E pipeline to run smoke/reliable tests on code changes so that we have at least some coverage (and because it doesn't create as much demand on CI resources as package-and-test - fewer than 5 jobs compared to 15+).

Part of gitlab-org/quality/quality-engineering&4 (closed)

Results with different code changes

Change	Auto/Excluded	Pipeline
CI change (this MR)	Auto ✅	https://gitlab.com/gitlab-org/gitlab/-/pipelines/834464480
QA change	Auto ✅	https://gitlab.com/gitlab-org/gitlab/-/pipelines/834505991
Non-QA change	Auto ✅	https://gitlab.com/gitlab-org/gitlab/-/pipelines/834511504
Docs change	Excluded ✅	https://gitlab.com/gitlab-org/gitlab/-/pipelines/834517902

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.