test: fix match when 3.15 produces better diagnostic.

Don't match exact ValueError, 3.15 produces: 'stray % at position 12'
while earlier versions reported: 'incomplete format'

So verify message up to ValueError only.

Author: rouilj

test/test_cgi.py

@@ -3714,8 +3714,7 @@ def testExpandfile(self):
             r"^Found an incorrect token when expandfile applied "
             r"string subsitution on "
             r"'[^']*[\\/]_test_template[\\/]html[\\/]file_with_broken_expand_type.js'. "
-            r"ValueError\('incomplete format'\) was raised. Check the format "
-            r"of your named conversion specifiers."))
+            r"ValueError"))
         self._caplog.clear()
         r = None