Introduced @RandomlyFails annotation to make it easier to skip unreliable functional tests.

First tried using excludedGroups and trying to work around SUREFIRE-984.
Made some progress, but not ideal since marked tests are omitted from output, rather than being listed as ignored.
Throwing AssumptionViolatedException from JenkinsRule is much simpler and seems to work better.
https://github.com/junit-team/junit/issues/116 notes its limitations.