Added py_test rule to Bazel

py_test rule enable to use a test written in Python. A py_test is
basically a py_binary that returns a non null on failure. Extraneous
support is need to have nice output (see //src/test/shell/unittest.bash
for the kind of support neeeded).

Actually the py_test code was already there but it was just missing the
necessary glue code. Also added an integration test for py_* rules in
Bazel.

--
MOS_MIGRATED_REVID=91407748
diff --git a/examples/py_native/fail.py b/examples/py_native/fail.py
new file mode 100644
index 0000000..98e35f4
--- /dev/null
+++ b/examples/py_native/fail.py
@@ -0,0 +1,13 @@
+"""A tiny example binary for the native Python rules of Bazel."""
+import unittest
+from examples.py_native.lib import GetNumber
+
+
+class TestGetNumber(unittest.TestCase):
+
+  def test_fail(self):
+    self.assertEquals(GetNumber(), 0)
+
+
+if __name__ == '__main__':
+  unittest.main()