Positive-path multiplex regression test, complementing
pmc_mux_test.c (which only exercises ENOSPC):

Build a (cores + 2)-event group with PMC_F_GROUP_MUX, attach all
siblings, start, run a CPU-busy child for 200 ms, stop, read every
sibling. Asserts:

• commit succeeds (no ENOSPC -- multiplex fallback engaged);
• every sibling has a non-zero count (group was scheduled in at least once);
• within-group atomicity is sound (max/min sibling-delta ratio stays bounded; rate variance across event types is allowed up to 64x);
• a second over-subscribed group running in parallel converges to roughly fair share (cross-group max-delta ratio <= 64x).

Like pmc_group_test, it is x86-only and skips itself on hosts
without AMD core PMCs.

Sponsored by: AMD
Signed-off-by: Raghavendra K T <raghavendra.kt@amd.com>