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/misc/misctests.cpp

@@ -125,6 +125,8 @@ void MiscTestCase::Delete()
 namespace
 {
 
+#ifdef __WXDEBUG__
+
 // helper function used just to avoid warnings about value computed not being
 // used in WX_ASSERT_FAILS_WITH_ASSERT() in StaticCast() below
 bool IsNull(void *p)
@@ -132,6 +134,8 @@ bool IsNull(void *p)
     return p == NULL;
 }
 
+#endif // __WXDEBUG__
+
 } // anonymous namespace
 
 void MiscTestCase::StaticCast()