x86: create pmu group for quick pmu-scope testing

Any agent can run "./run_tests.sh -g pmu" to run all PMU tests easily,
e.g. when verifying the x86/PMU KVM changes.

Signed-off-by: Like Xu <likexu@tencent.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221102225110.3023543-10-seanjc@google.com