jenkins distcheck: enable external tests

It was recently discovered that logging_vty_test.vty was missing from
EXTRA_DIST. To make sure we don't forget similar files in the future, add
python tests to 'make distcheck', by means of DISTCHECK_CONFIGURE_FLAGS.

Related: I1bcedf3097f02b2adc679560d1cbceb27dbc345e
Change-Id: Id569b2a932c1428cabb4d7ff17822cff8eee02af
diff --git a/contrib/jenkins_amd64.sh b/contrib/jenkins_amd64.sh
index cd7b666..9b83793 100755
--- a/contrib/jenkins_amd64.sh
+++ b/contrib/jenkins_amd64.sh
@@ -28,7 +28,8 @@
 
 # do distcheck only once, which is fine from built source tree, since distcheck
 # is well separated from the source tree state.
-$MAKE distcheck \
+DISTCHECK_CONFIGURE_FLAGS=--enable-external-tests \
+    $MAKE distcheck \
     || cat-testlogs.sh
 
 osmo-clean-workspace.sh