Add a README for tests/end2end/data/hints/html

See #1542

tests/end2end/data/hints/html/README.md

@@ -0,0 +1,5 @@
+Tests in this directory are automatically picked up by `test_hints` in
+`tests/end2end/test_hints_html.py`.
+
+They need to contain a special `<!-- target: foo.html -->` comment which
+specifies where the hint in it will point to, and will then test that.

tests/end2end/test_hints_html.py

@@ -31,7 +31,7 @@ import textwrap
 def collect_tests():
     basedir = os.path.dirname(__file__)
     datadir = os.path.join(basedir, 'data', 'hints', 'html')
-    files = os.listdir(datadir)
+    files = [f for f in os.listdir(datadir) if f != 'README.md']
     return files