[Pipeline] Check pipeline-test-ydbtest.csh against all YDB MRs not just open ones

This commit is a follow-up to commit efe7819f.

We found out that in our workflow, we frequently merge the YDB MR PRIOR to creating the YDBTest MR; and thus the pipeline would fail because it can't find the YDB MR. This commit changes the behavior of the pipeline to look for ALL recent YDB MRs, rather than only those that are open.

Brief testing shows that the API returns by default the last 20 MRs in reverse chronological order. The last MR created shows up first.

Note that a successful pipeline still requires that the author does not delete their branch.