test-manifest: make manifesttest a base class that is extended

The implementation of the testmanifest test case is moved to a new base class,
which is then extended to make the testmanifest. And instead of testmanifest,
the subclass is named testmanifestdict because, well, that's what it's testing.

This refactoring makes it possible to create alternate versions of what was
formerly testmanifest, improving test coverage of different manifestdict
implementations.

tests/test-manifest.py

@@ -95,9 +95,9 b" A_HUGE_MANIFEST = ''.join(sorted("
                    itertools.cycle((HASH_1, HASH_2)),
                    itertools.cycle(('', 'x', 'l')))))
 
-class testmanifest(unittest.TestCase):
+class basemanifesttests(object):
     def parsemanifest(self, text):
-        return manifestmod.manifestdict(text)
+        raise NotImplementedError('parsemanifest not implemented by test case')
 
     def assertIn(self, thing, container, msg=None):
         # assertIn new in 2.7, use it if available, otherwise polyfill
@@ -456,5 +456,9 b' class testmanifest(unittest.TestCase):'
                 ['a/b/c/bar.txt', 'a/b/c/foo.txt', 'a/b/d/ten.txt'],
                 m2.keys())
 
+class testmanifestdict(unittest.TestCase, basemanifesttests):
+    def parsemanifest(self, text):
+        return manifestmod.manifestdict(text)
+
 if __name__ == '__main__':
     silenttestrunner.main(__name__)