Move remaining *.shader_test files out of 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 MRs adding tests to the legacy tests/shaders directory. This series moves all the remaining *.shader_test files out of the tests/shaders directory.