perf test attr: Relax group checking for ungrouped expectations

When matching the recorded events back against the expectations,
attr.py requires every event opened in a group to match an expectation
that declares the same group.  The perf stat default metrics open the
events of each metric in groups, but the test expectations only verify
the event attributes, so test-stat-default fails with:

  FAILED '.../test-stat-default' - group failure

Only enforce the group of a recorded event when the expectation it
matches declares one.  Expectations that ask for a group are still
verified in both directions.

Fixes: c7adeb0974f18da4 ("perf jevents: Add set of common metrics based on default ones")
Fixes: 2dfc0cab3da4a205 ("perf stat: Add detail -d,-dd,-ddd metrics")
Assisted-by: LLM
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>