Run builds in tests on agents by default even when the test does not specifically request this

[jglick]

Prototype of a test facility useful for catching mistakes of many types. Useful during jenkinsci/jenkins#5880 research. Needs more work to make this opt-in; and more reliable: will cause spurious failures for various reasons:

• Various failures in e.g. workflow-basic-steps or credentials-binding tests as discussed in comments in sources; tricky to fix.
• GUI tests which expect only one distinct Cloud to be present (e.g. ec2 or kubernetes).
• Tests expecting something to run on master without using a label, e.g. AntTest.testParameterExpansion or FreestyleJobTest.customWorkspace or MatrixProjectCustomWorkspaceTest.customWorkspaceForParent, or some other cloud without a label.