Tidy up the tests/shaders directory

tests/shaders was historically where any shader related test was dumped when shader support was first added to piglit. Since then we tend to put tests in the much more organised tests\spec directory.

Unfortunately just about every new contributor to piglit submits new shader MR adding tests to the legacy tests/shaders directory. This series takes a big step towards cleaning up the directory once and for all (with the goal of eventually removing it).