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.