test-manifest: add some test coverage for treemanifest

Similar to the testmanifest test case, testtreemanifest extends the base test
case but uses treemanifests instead of manifestdicts. Adding this test provides
some basic test coverage of treemanifest within the standard test suite.

tests/test-manifest.py

@@ -460,5 +460,9 b' class testmanifestdict(unittest.TestCase'
     def parsemanifest(self, text):
         return manifestmod.manifestdict(text)
 
+class testtreemanifest(unittest.TestCase, basemanifesttests):
+    def parsemanifest(self, text):
+        return manifestmod.treemanifest('', text)
+
 if __name__ == '__main__':
     silenttestrunner.main(__name__)