tests/pm_pc8: rename some subtests
tests/pm_pc8: rename some subtests

Due to copy/paste error, we had 3 subtests called gem-execbuf-stress.

Signed-off-by: Paulo Zanoni <cc0e04a2103c45cd195651d976f79813d0f66bdf@intel.com>

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