tests: opt into python3 when testing virtualenv installation

Otherwise it complains that Mercurial requires '~=2.7'.  Since the existing
linux py3 test doesn't flag this, I'm assuming that virtualenv simply isn't
installed on that system.

Differential Revision: https://phab.mercurial-scm.org/D7111

tests/test-install.t

@@ -236,6 +236,11 b' Verify the json works too:'
 
 #endif
 
+#if py3
+  $ HGALLOWPYTHON3=1
+  $ export HGALLOWPYTHON3
+#endif
+
 #if virtualenv
 
 Verify that Mercurial is installable with pip. Note that this MUST be