test-highlight: add normalization rule for Pygments 2.1

It appears that several classes have 'h', '1', '2' suffixes on Pygments 2.1,
which is the current version on Debian sid.

tests/test-highlight.t

@@ -12,7 +12,8 b''
 
   $ filterhtml () {
   >   sed -e "s/class=\"k\"/class=\"kn\"/g" \
-  >       -e "s/class=\"mf\"/class=\"mi\"/g"
+  >       -e "s/class=\"mf\"/class=\"mi\"/g" \
+  >       -e "s/class=\"\([cs]\)[h12]\"/class=\"\1\"/g"
   > }
 
 create random Python file to exercise Pygments