Allow for running extended tests with a focus

[miminar]

I'd really like to run registry and pruning related tests for registry code changes like before:

[testextended][extended:core(ImagePrun|registry)]

[miminar]

/cc @Kargakis

[stevekuznetsov]

This is probably not going to happen. What we can do is write a non-blocking job /test registry or something that you guys can use that encodes a focus you use often -- can you provide one? I'll make the job

[miminar]

@stevekuznetsov My favorite focus is 'ImagePrune|registry|ImageLookup|ImageQuota'. It could be simplified to Image|registry.

[stevekuznetsov]

@miminar ask and you shall receive :)

[miminar]

Thanks man! Resolved.