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/uris/url.cpp

@@ -123,7 +123,9 @@ void URLTestCase::CopyAndAssignment()
     CPPUNIT_ASSERT(url1 == url2);
 
     // assignment to self
+    wxCLANG_WARNING_SUPPRESS(self-assign-overloaded)
     url2 = url2;
+    wxCLANG_WARNING_RESTORE(self-assign-overloaded)
 
     // check for destructor (with base pointer!)
     puri = new wxURL();