run-tests.py: inherit --pure option from outer run-tests.py execution

Before this patch, "test-run-tests.t" doesn't test "run-tests.py" with
"--pure", even if outer "run-tests.py" is executed with it.

This patch uses not "HG_RUN_TESTS_PURE" but "HGTEST_RUN_TESTS_PURE",
because "HG_" prefixed environments are forcibly dropped in "_getenv()".

This is also useful to run "run-tests.py" successfully by
"run-tests.py --pure" on Windows without any compilation tools (like
VisualStudio).

tests/run-tests.py

@@ -1664,6 +1664,9 b' class TestRunner(object):'
             pypath.append(oldpypath)
         os.environ[IMPL_PATH] = os.pathsep.join(pypath)
 
+        if self.options.pure:
+            os.environ["HGTEST_RUN_TESTS_PURE"] = "--pure"
+
         self._coveragefile = os.path.join(self._testdir, '.coverage')
 
         vlog("# Using TESTDIR", self._testdir)

tests/test-run-tests.t

@@ -3,7 +3,7 b' This file tests the behavior of run-test'
 Smoke test
 ============
 
-  $ $TESTDIR/run-tests.py
+  $ $TESTDIR/run-tests.py $HGTEST_RUN_TESTS_PURE
   
   # Ran 0 tests, 0 skipped, 0 warned, 0 failed.