i915/gem_exec_basic: Use dynamic subtests

Instead of generating a subtest for each engine in a static list,
convert to dynamic subtests, with one dynamic subtest per actually
present physical engine.

v2:
 - Remove the now-redundant *-all subtests
 - Use the "basic" subtest in intel-ci/fast-feedback.testlist

Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Reviewed-by: Katarzyna Dec <katarzyna.dec@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>