Distcheck repair: add new python testing code to "tarballs"

'make distcheck' now passes again.

src/test/include.am

@@ -108,3 +108,9 @@ endif
 	./src/test/test-bt-cl assert | $(PYTHON) $(top_srcdir)/src/test/bt_test.py
 	./src/test/test-bt-cl crash | $(PYTHON) $(top_srcdir)/src/test/bt_test.py
 endif
+
+EXTRA_DIST += \
+	src/test/bt_test.py \
+	src/test/ntor_ref.py \
+	src/test/slownacl_curve25519.py \
+	src/test/test_cmdline_args.py