Advise against removing run-make/libtest-junit/validate_junit.py

Trying to get rid of this Python script looks tempting, because it's currently
the only Python script in the whole `run-make` suite that we actually run.

But getting rid of it would require pulling in a Rust crate to parse XML
instead, and that's probably not worth the extra hassle for a relatively-minor
test.

tests/run-make/libtest-junit/validate_junit.py

@@ -1,5 +1,15 @@
 #!/usr/bin/env python
 
+# Trivial Python script that reads lines from stdin, and checks that each line
+# is a well-formed XML document.
+#
+# This takes advantage of the fact that Python has a built-in XML parser,
+# whereas doing the same check in Rust would require us to pull in an XML
+# crate just for this relatively-minor test.
+#
+# If you're trying to remove Python scripts from the test suite, think twice
+# before removing this one. You could do so, but it's probably not worth it.
+
 import sys
 import xml.etree.ElementTree as ET