Integrate revng-distributable tests with orchestra

[fcremo]

This PR integrates revng-distributable tests with orchestra. To run the tests, do orc install -b --test revng-distributable. By default, tests run on all supported distros using podman and xvfb. To change this, add the following to user_options.yml:

#@overlay/replace
cold_revng_test_method: host-x11

You can see the list of supported testing modes in revng_distributable.yml.

The PR also includes some commits with misc improvements and fixes:

• use orc inspect to list files to copy instead of assuming orchestra writes a .idx file in share/orchestra
• bump xcbproto version to fix a build error due to usage of a deprecated function in a python script
• fix a bug in libglvnd which caused it to not find a dynamic library
• fix QT using the host libmd4c instead of its vendored version