testsuite: run C++11 tests in C++11 mode

A recent change missed updating constexpr-mutable3.C because it wasn't run
in C++11 mode even though it checks the behavior for { target c++11_only }.

gcc/testsuite/ChangeLog:

	* lib/g++-dg.exp (g++-dg-runtest): Check for c++11_only.