Negative-path multiplex regression test: build a (cores + 1)-event
group with no PMC_F_GROUP_MUX, expect commit to return ENOSPC.
Sized off probe_core_pmcs() so it adapts to any AMD Zen variant
(6 PMCs on Zen2/3/4, 6 on Zen5 core, etc.).
Sponsored by: AMD
Signed-off-by: Raghavendra K T <raghavendra.kt@amd.com>