tests: $(BUILT_SOURCES) is not defined, depend on osmo-mgw

Change-Id: I951ce2410614993a855336e6bb408cae1823ef9a
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 49a659f..745afd6 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -34,11 +34,11 @@
 	$(NULL)
 
 if ENABLE_EXT_TESTS
-python-tests: $(BUILT_SOURCES)
+python-tests: $(top_builddir)/src/osmo-mgw/osmo-mgw
 	osmotestvty.py -p $(abs_top_srcdir) -w $(abs_top_builddir) -v
 	osmotestconfig.py -p $(abs_top_srcdir) -w $(abs_top_builddir) -v
 else
-python-tests: $(BUILT_SOURCES)
+python-tests:
 	echo "Not running python-based tests (determined at configure-time)"
 endif