Fix tests build with wxDEBUG_LEVEL==0

Define various macros used in the test code (or in the headers included
from it) as nothing when wxDEBUG_LEVEL==0.

Also don't define a helper function used when asserts are enabled in
this build to avoid clang warnings about unused function.

tests/testprec.h

@@ -128,6 +128,7 @@ public:
     // normal build with wxDEBUG_LEVEL != 0 we can pass something not
     // evaluating to a bool at all but it then would fail to compile in
     // wxDEBUG_LEVEL == 0 case, so just don't do anything at all now).
+    #define WX_ASSERT_FAILS_WITH_ASSERT_MESSAGE(msg, code)
     #define WX_ASSERT_FAILS_WITH_ASSERT(cond)
 #endif