tests/flip_test: new testcase with a busy load before the flip
tests/flip_test: new testcase with a busy load before the flip

I've hoped that this would be good enough to hang pch platforms, but
it looks like those just complete the flip immediately on disabled pipes.

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