Separate mechanism from policy: PreCommit is policy, the set of tests is a named suite

Throughout our build.gradle files we have sprinkled "preCommit" tasks. It obscures what is run sometimes.

One consistent way of managing this is to have modules define test suites and to have centralized management of which suites are pre/post commit, defined entirely in the root build.gradle. We are almost doing this already, except for...

The other way of doing it which is to let modules request which suites should be pre/post commits and the root build.gradle is expected to call those tasks.

It isn't really clear what the intent of our tasks are right now, to me anyhow. I think they've organically grown and could now be put in order a bit.

Imported from Jira [BEAM-5847](https://issues.apache.org/jira/browse/BEAM-5847). Original Jira may contain additional context.
Reported by: kenn.