Clean up completion tests. Use of conslidated helper may make
instrumenting one particular test during debugging of the test
itself, but I think that issue should be addressed in some other
way (e.g. making sure individual tests in 9902 can be skipped).
* fc/completion-test-simplification:
completion: simplify __gitcomp() test helper
completion: refactor __gitcomp related tests
completion: consolidate test_completion*() tests
completion: simplify tests using test_completion_long()
completion: standardize final space marker in tests
completion: add comment for test_completion()