Add WX_ASSERT_FAILS_WITH_ASSERT_MESSAGE helper.

This is similar to the existing WX_ASSERT_FAILS_WITH_ASSERT, but allows to
specify the message to give if the expected assertion is not triggered.

tests/testprec.h

@@ -78,14 +78,19 @@ public:
 #if wxDEBUG_LEVEL
     // some old cppunit versions don't define CPPUNIT_ASSERT_THROW so roll our
     // own
-    #define WX_ASSERT_FAILS_WITH_ASSERT(cond) \
-        { \
+    #define WX_ASSERT_FAILS_WITH_ASSERT_MESSAGE(msg, code) \
+        wxSTATEMENT_MACRO_BEGIN \
             bool throwsAssert = false; \
-            try { cond ; } \
+            try { code ; } \
             catch ( const TestAssertFailure& ) { throwsAssert = true; } \
             if ( !throwsAssert ) \
-                CPPUNIT_FAIL("expected assertion not generated"); \
-        }
+                CPPUNIT_FAIL(msg); \
+        wxSTATEMENT_MACRO_END
+
+    #define WX_ASSERT_FAILS_WITH_ASSERT(code) \
+        WX_ASSERT_FAILS_WITH_ASSERT_MESSAGE( \
+            "expected assertion not generated", code)
+
 #else
     // there are no assertions in this build so we can't do anything (we used
     // to check that the condition failed but this didn't work well as in