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

tools/python/BUILD

diff --git a/tools/python/BUILD b/tools/python/BUILD
index f489fe1..6933f9d 100644
--- a/tools/python/BUILD
+++ b/tools/python/BUILD
@@ -4,3 +4,12 @@
     name = "2to3",
     srcs = ["2to3.sh"],
 )
+
+filegroup(
+    name = "srcs",
+    srcs = [
+        # Tools are build from the workspace for tests.
+        "2to3.sh",
+        "BUILD",
+    ],
+)