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.