Previously, the test runner would only accept leading options,
an optional "--" separator, followed by "$TEST" and optional arguments
for the test.

Like

  ./tools/run-nm-test.sh -m src/settings/plugins/ifcfg-rh/tests/test-ifcfg-rh

That is annoying, because to toggle an option you have to seek the
curser in the before the test name.

Now, accept a "--test" option, so that the above can be done with trialing
arguments:

  ./tools/run-nm-test.sh -t src/settings/plugins/ifcfg-rh/tests/test-ifcfg-rh -m

However, the arguments for the tests still must come last:

  ./tools/run-nm-test.sh -m [--] src/settings/plugins/ifcfg-rh/tests/test-ifcfg-rh -p /settings/plugins/ifcfg-rh/bridge/write-master
  ./tools/run-nm-test.sh -t src/settings/plugins/ifcfg-rh/tests/test-ifcfg-rh -m [--] -p /settings/plugins/ifcfg-rh/bridge/write-master