gtestutils: Define a custom g_print handler for TAP

When using TAP output in gtest, all the printed strings should be commented
not to break the spec, so enforce this at GTest level, by ensuring that
all the lines written via g_print() will be in the commented form and will
respect the subtest indentation.

As per this we can remove some custom code in g_test_log() as now there are
very few cases in which we need to use the default print handler which is
now private when testing.

glib/tests/testing-helper.c

@@ -93,6 +93,16 @@ test_message (void)
   g_test_message ("\nTests that multi\nline\nmessage\nworks with leading and trailing too\n");
 }
 
+static void
+test_print (void)
+{
+  g_print ("Tests that single line message works\n");
+  g_print ("test that multiple\nlines ");
+  g_print ("can be ");
+  g_print ("written ");
+  g_print ("separately\n");
+}
+
 int
 main (int   argc,
       char *argv[])
@@ -212,6 +222,10 @@ main (int   argc,
     {
       g_test_add_func ("/message", test_message);
     }
+  else if (g_strcmp0 (argv1, "print") == 0)
+    {
+      g_test_add_func ("/print", test_print);
+    }
   else
     {
       g_assert_not_reached ();