x86/pmu: Run the "emulation" test iff forced emulation is available

Run the PMU's emulation testcase if and only if forced emulation is
available, and do so without requiring the user to manually specify they
want to run the emulation testcase.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220808164707.537067-8-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>