drm/i915/selftests: Update live.evict to wait on requests / idle GPU after each loop

Update live.evict to wait on last request and idle GPU after each loop.
This not only enhances the test to fill the GGTT on each engine class
but also avoid timeouts from igt_flush_test when using GuC submission.
igt_flush_test (idle GPU) can take a long time with GuC submission if
losts of contexts are created due to H2G / G2H required to destroy
contexts.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211021214040.33292-1-matthew.brost@intel.com