check-code: allow only-whitespace lines in tests

These turn out to be necessary to embed diffs with blank lines in test
input. So we now complain about whitespace following a character.

contrib/check-code.py

@@ -148,7 +148,7 b' winglobmsg = "use (glob) to match Window'
 uprefix = r"^  \$ "
 utestpats = [
   [
-    (r'^(\S.*||  [$>] .*)[ \t]\n', "trailing whitespace on non-output"),
+    (r'^(\S.*||  [$>] \S.*)[ \t]\n', "trailing whitespace on non-output"),
     (uprefix + r'.*\|\s*sed[^|>\n]*\n',
      "use regex test output patterns instead of sed"),
     (uprefix + r'(true|exit 0)', "explicit zero exit unnecessary"),