tools: add a test program to parse the commandline options

tools: add a test program to parse the commandline options

A pytest wrapper around our xkbcli tool - copied from libinput.
This calls our various xkbcli tools with varying options and check that they
either succeed or return the right error code. The coverage is limited, it
does not (and cannot) test for all possible combinations but it should provide a
good red flag if we have inconsistent behavior or accidentally break some
combination of flags.

Meanwhile, we can at least assume that all our commandline arguments are parsed
without segfaulting or worse.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>