mkswap: remove unnecessary usage output

The usage output changes too often making unnecessarily expected output
in test unstable.  Besides in this case the usage ouput does not even
help user, as use of any of the options will not solve the problem.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
tests: check swaplabel, and perform more swapon boundary tests

Signed-off-by: Sami Kerola <kerolasa@iki.fi>