The flip6 build profile only runs the Flip-6 related test cases. Moreover,
all Flip-6 related test cases are excluded when not running the flip6 build
profile. This should reduce testing time when adding more and more Flip-6
test cases.

Include flink-test-utils-junit in all submodules to make the Category marker interfaces Flip6 and OldAndFlip6 available

This closes #4889.