Suppress harmless warnings about string literals in test code

We rely on string literals being of non-const char or wchar_t pointer type for
this code to compile, even if this results in warnings, so we're not
interested in these warnings in the test code itself.

tests/strings/vararg.cpp

@@ -101,9 +101,16 @@ void VarArgTestCase::StringPrintf()
     CPPUNIT_ASSERT( s2 == "value=FooBar;" );
 
     // this tests correct passing of wxCStrData constructed from string
-    // literal:
+    // literal (and we disable the warnings related to the use of a literal
+    // here because we want to test that this compiles, even with warnings):
+    wxGCC_WARNING_SUPPRESS(write-strings)
+    wxCLANG_WARNING_SUPPRESS(c++11-compat-deprecated-writable-strings)
+
     bool cond = true;
     s2.Printf(wxT("foo %s"), !cond ? s.c_str() : wxT("bar"));
+
+    wxGCC_WARNING_RESTORE(write-strings)
+    wxCLANG_WARNING_RESTORE(c++11-compat-deprecated-writable-strings)
 }
 
 void VarArgTestCase::CharPrintf()