run-tests: set HGMODULEPOLICY for --pure

Without this, my python 2.6 virtualenv test run with --pure and
--local fails with:

+  ImportError: Python minor version mismatch: The Mercurial extension modules were compiled with Python 2.7.8, but Mercurial is currently using Python with sys.hexversion=33950192: Python 2.6.9 (unknown, Apr 13 2016, 12:40:12)
+  [GCC 4.9.2 20141101 (Red Hat 4.9.2-1)]
+   at: ~/hg/py26/bin/python

tests/run-tests.py

@@ -2115,6 +2115,7 b' class TestRunner(object):'
 
         if self.options.pure:
             os.environ["HGTEST_RUN_TESTS_PURE"] = "--pure"
+            os.environ["HGMODULEPOLICY"] = "py"
 
         if self.options.allow_slow_tests:
             os.environ["HGTEST_SLOW"] = "slow"