tests: add a test to compile all xkb_symbols section with xkbcomp

Using pytest because it does a lot of the setup/tracing stuff for us. The test checks all symbols files and generates a list of tuples (layout, variant) from the files. That's then used in a generic-enough keymap to be fed to xkbcomp. Where xkbcomp fails we fail the test (and pytest will collect stdout/stderr/etc.) for us.