Added test for Mercurial fetching.

var/spack/mock_packages/hg-test/package.py

@@ -0,0 +1,10 @@
+from spack import *
+
+class HgTest(Package):
+    """Test package that does fetching with mercurial."""
+    homepage = "http://www.hg-fetch-example.com"
+
+    version('hg', hg='to-be-filled-in-by-test')
+
+    def install(self, spec, prefix):
+        pass