drm/i915/selftests: Use less in contexts steal guc id test

Using more guc_ids in the stealing guc id test has no real benefit.
Tearing down lots of contexts all at the same time takes a bit of time
due to the H2G / G2H ping-pong with the GuC. On some slower platforms
this can cause timeous when flushing the test as the GT isn't idle when
this ping-pong is happening. Reduce the number of guc ids to speed up
the flushing of the test.

Link: https://gitlab.freedesktop.org/drm/intel/-/issues/4821
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220122000822.25616-1-matthew.brost@intel.com