tests: Use argparse module with hwsim

tests: Use argparse module with hwsim

Instead of re-implementing a command-line parser, use the
argparse module.

The only real change (I hope) is that the test module must
now be given to the -f option without the .py suffix.

Also, --help now works, and if a test module/test name is
given that doesn't exist, the valid list is printed.

Signed-hostap: Johannes Berg <johannes.berg@intel.com>