only check testgrid runs for the latest commit in a branch

[laverya]

What this PR does / why we need it:

At present, if any testgrid runs for a PR have failed, the testgrid checker will report failure. Even if that has been fixed in a later commit.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Steps to reproduce

Does this PR introduce a user-facing change?

Does this PR require documentation?