docs: limit the number of subtests listed
docs: limit the number of subtests listed

Avoid producing long lists of subtests in the documentation and instead
provide instructions on how to obtain the full list.

Signed-off-by: Thomas Wood <b9025d6a953b68f894a0f6a80fd2af6d55b0badd@intel.com>

tiagovignatti/intel-gpu-tools,tiagovignatti/intel-gpu-tools,dlespiau/checkmate-test-igt,dlespiau/checkmate-test-igt,mv0/intel-gpu-tools,chenxianqin/intel-gpu-tools,dlespiau/checkmate-test-igt,chenxianqin/intel-gpu-tools,mv0/intel-gpu-tools,mv0/intel-gpu-tools,chenxianqin/intel-gpu-tools,mv0/intel-gpu-tools,tiagovignatti/intel-gpu-tools,chenxianqin/intel-gpu-tools,dlespiau/checkmate-test-igt,tiagovignatti/intel-gpu-tools