Add tests for flag completion registration (#2053)

Different problems have been reported about flag completion registration.
These two tests are the cases that were not being verified but had been
mentioned as problematic.

Ref:
- https://github.com/spf13/cobra/issues/1320
- https://github.com/spf13/cobra/pull/1438#issuecomment-872928669

Signed-off-by: Marc Khouzam <marc.khouzam@gmail.com>