add test that checks README.rst against usbsdmux -h output

[jluebbe]

The usbsdmux -h output in the README.rst has become out of date with the actual output of the tool. This can be quite confusing, as it makes it unclear which features the tool has.

Update the README.rst content and make sure it stays up to date via a pytest test case.

This is an alternative to #64. Instead of adding a custom Actions workflow for a special check, integrate it as a test into the pytest testsuite. This also gives us the diff output through assertions (see https://github.com/jluebbe/usbsdmux/actions/runs/7288823489/job/19862144881#step:4:84 as an example).

[jluebbe]

I had the simple variant first as well, but found the diff output was worth the additional code.