Set the correct C++ standard version so that test pass with GCC 11

This should address issue:

diffblue#6090

Author: martin

regression/cpp/Method_qualifier1/test.desc

@@ -1,6 +1,6 @@
 CORE winbug macos-assert-broken
 main.cpp
-
+-std=c++14
 ^EXIT=0$
 ^SIGNAL=0$
 --

regression/cpp/virtual1/test.desc

@@ -1,6 +1,6 @@
 CORE macos-assert-broken
 main.cpp
-
+-std=c++14
 ^EXIT=0$
 ^SIGNAL=0$
 --