Add comment about testing with reviewable.

tools/run_tests/jobset.py

@@ -50,6 +50,10 @@ _DEFAULT_MAX_JOBS = 16 * multiprocessing.cpu_count()
 _MAX_RESULT_SIZE = 8192
 
 
+# NOTE: If you change this, please make sure to test reviewing the
+# github PR with http://reviewable.io, which is known to add UTF-8
+# characters to the PR description, which leak into the environment here
+# and cause failures.
 def strip_non_ascii_chars(s):
   return ''.join(c for c in s if ord(c) < 128)