x86/pmu: Update rdpmc testcase to cover #GP path

Specifying an unsupported PMC encoding will cause a #GP(0).

There are multiple reasons RDPMC can #GP, the one that is being relied
on to guarantee #GP is specifically that the PMC is invalid. The most
extensible solution is to provide a safe variant.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Like Xu <likexu@tencent.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221102225110.3023543-12-seanjc@google.com