KVM: selftests: Split PMU caps sub-tests to avoid writing MSR after KVM_RUN

Split the PERF_CAPABILITIES subtests into two parts so that the LBR format
testcases don't execute after KVM_RUN.  Similar to the guest CPUID model,
KVM will soon disallow changing PERF_CAPABILITIES after KVM_RUN, at which
point attempting to set the MSR after KVM_RUN will yield false positives
and/or false negatives depending on what the test is trying to do.

Land the LBR format test in a more generic "immutable features" test in
anticipation of expanding its scope to other immutable features.

Link: https://lore.kernel.org/r/20230311004618.920745-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>