Explain the run-make/debugger-visualizer-dep-info Python script

Author: Zalathar

tests/run-make/debugger-visualizer-dep-info/foo.py

@@ -1 +1,6 @@
-# empty
+# This is a Python script, but we don't actually run it.
+# So if you're trying to remove Python scripts from the test suite,
+# be aware that there's no value in trying to get rid of this one.
+#
+# It just needs to exist so that the compiler can embed it via
+# `#![debugger_visualizer(gdb_script_file = "...")]`.