Suppress some harmless clang warnings in the test suite

Use wxCLANG_WARNING_SUPPRESS() to avoid multiple warnings in CATCH
headers as well as in some of our own tests.

tests/weakref/weakref.cpp

@@ -133,7 +133,9 @@ void WeakRefTestCase::DeclareTest()
         wxWeakRef<IncompleteClass> p;
 
         // Copying should be also OK
+        wxCLANG_WARNING_SUPPRESS(self-assign-overloaded)
         p = p;
+        wxCLANG_WARNING_RESTORE(self-assign-overloaded)
 
         // Assigning a raw pointer should cause compile error
 #ifdef TEST_INVALID_INCOMPLETE_WEAKREF