Put the TODO comment in a YAML array.

The `contains:` map needs an array of strings.  By placing the TODO comment between brackets I hope that the user will be more likely to produce a valid file. 

Closes: nf-core#1082

nf_core/modules/test_yml_builder.py

@@ -241,7 +241,7 @@ def get_md5_sums(self, entry_point, command, results_dir=None, results_dir_repea
                 test_files[i].pop("md5sum")
                 test_files[i][
                     "contains"
-                ] = "# TODO nf-core: file md5sum was variable, please replace this text with a string found in the file instead"
+                ] = "[ # TODO nf-core: file md5sum was variable, please replace this text with a string found in the file instead ]"
 
         if len(test_files) == 0:
             raise UserWarning(f"Could not find any test result files in '{results_dir}'")