tests: $(BUILT_SOURCES) is not defined

Change-Id: I81dc30f6d6e3b25723f00d945c87b9c01c648cbf
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 720d631..5084704 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -21,11 +21,12 @@
 TESTSUITE = $(srcdir)/testsuite
 
 if ENABLE_EXT_TESTS
-python-tests: $(BUILT_SOURCES)
+python-tests: $(top_builddir)/src/osmo-pcap-server \
+	      $(top_builddir)/src/osmo-pcap-client
 	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