Fix building tests with wxUSE_UNSAFE_WXSTRING_CONV==0

Avoid relying on implicit conversions to std::string in the tests code.

tests/strings/strings.cpp

@@ -1028,8 +1028,10 @@ void StringTestCase::CStrDataImplicitConversion()
     // implicit conversion of wxString is not available in STL build
 #if !wxUSE_STL
     CPPUNIT_ASSERT( CheckStrConstWChar(s, s) );
+#if wxUSE_UNSAFE_WXSTRING_CONV
     CPPUNIT_ASSERT( CheckStrConstChar(s, s) );
 #endif
+#endif
 }
 
 void StringTestCase::ExplicitConversion()