selftests: kvm: Merge user_msr_test into userspace_msr_exit_test

Both user_msr_test and userspace_msr_exit_test tests the functionality
of kvm_msr_filter.  Instead of testing this feature in two tests, merge
them together, so there is only one test for this feature.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Message-Id: <20201204172530.2958493-1-aaronlewis@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>