prow: add a test to blocker list per-PR when the test is triggered manually

[stevekuznetsov]

It would be cool if, when a PR has a non-blocking test triggered for it manually, that either this test would automatically become part of the default trigger set and become blocking for the PR or if there were a bot command to add it to the list as well.

/area prow
/cc @Kargakis @bparees

[0xmichalis]

Seems like a munger issue. The easiest way I can think of is the SQ blocks on the context if the context exists in the PR, otherwise it is not.

/area mungegithub

[spxtr]

@rmmh I think was interested in this.

[stevekuznetsov]

Also necessary if a test gets selected by touching changed paths

[0xmichalis]

Dupe of #1351