run_tests: allow specifying desired tests on the command line

This is a simpler version of Sean Christopherson's patch, which also allows
specifying more than one test.

Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>
Tested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>