radeonsi/tests: add a --slow option
Some glcts tests implement tons of tests because they verify
every possible combination of format/swizzle/target/...
They take a long time to execute and aren't possible to run
using multiple processes.
The proper way to fix it would be to split them in vk-gl-cts,
as is already done for some of them (eg es31fTextureGatherTests.cpp).
In the meantime, not running them makes glcts run almost
10 times faster.
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16580>