tests: Update document validation related to tests

If a test is not done, standardize on "TODO" as the only allowed
text. Otherwise, the txt is considered to be a valid test and an
error will be flagged if the test cannot be found in
layer_validation_tests.cpp.
diff --git a/vk_layer_documentation_generate.py b/vk_layer_documentation_generate.py
index 96eb3d5..7feeff6 100755
--- a/vk_layer_documentation_generate.py
+++ b/vk_layer_documentation_generate.py
@@ -348,9 +348,9 @@
                                 print(self.txt_color.yellow() + 'Validation check %s has missing or invalid test : %s' % (chk, test))
                                 warnings_found += 1
                                 break
-                    elif test not in tests_set and not chk.endswith('_NONE'):
-                        print(self.txt_color.yellow() + 'Validation check %s has missing or invalid test : %s' % (chk, test))
-                        warnings_found += 1
+                    elif test not in tests_set and not chk.endswith('_NONE') and test != 'TODO':
+                        print(self.txt_color.red() + 'Validation check %s has missing or invalid test : %s' % (chk, test))
+                        errors_found += 1
         # Now go through all of the actual checks in the layers and make sure they're covered in the doc
         for ln in layer_dict:
             for chk in layer_dict[ln]['CHECKS']: