It brings in a lot of feature such as segfault detection and backtrace printing
generation of xunit files, ability to run in gdb or valgrind
etc...

Remove the option to generate the xunit file as the user should simply
run gst-validate-launcher tests/run_testsuite.py --xunit-file=file.xunit
to generate it, no reason to have it in the build system anymore.

Differential Revision: https://phabricator.freedesktop.org/D1677