Ignore GTK debug logs in the unit test unless they're enabled

Don't output "*** GTK log message while running" messages for every
g_debug() call when we the debug messages themselves will not be
displayed because G_MESSAGES_DEBUG is not set or its value doesn't
include the domain used by the message.

This results in much more reasonable output from the test suite.

See #17400.

tests/test.cpp

@@ -490,6 +490,26 @@ wxTestGLogHandler(const gchar* domain,
                   const gchar* message,
                   gpointer data)
 {
+    // Check if debug messages in this domain will be logged.
+    if ( level == G_LOG_LEVEL_DEBUG )
+    {
+        static consr char* const allowed = getenv("G_MESSAGES_DEBUG");
+
+        // By default debug messages are dropped, but if G_MESSAGES_DEBUG is
+        // defined, they're logged for the domains specified in it and if it
+        // has the special value "all", then all debug messages are shown.
+        //
+        // Note that the check here can result in false positives, e.g. domain
+        // "foo" would pass it even if G_MESSAGES_DEBUG only contains "foobar",
+        // but such cases don't seem to be important enough to bother
+        // accounting for them.
+        if ( !allowed ||
+                (strcmp(allowed, "all") != 0 && !strstr(allowed, domain)) )
+        {
+            return;
+        }
+    }
+
     fprintf(stderr, "\n*** GTK log message while running %s(): ",
             wxGetCurrentTestName().c_str());