Fail CI pipeline when "make test" does not run any system tests

Apart from ensuring "make test" returns 0, also check whether any system
test output was generated as a result of running it.  This prevents the
CI job running system tests from succeeding unless it actually tests
something.

.gitlab-ci.yml

@@ -100,7 +100,8 @@ stages:
     - rm -rf .ccache
     - bash -x bin/tests/system/ifconfig.sh up
   script:
-    - cd bin/tests && make -j${TEST_PARALLEL_JOBS:-1} -k test V=1
+    - ( cd bin/tests && make -j${TEST_PARALLEL_JOBS:-1} -k test V=1 )
+    - test -s bin/tests/system/systests.output
   artifacts:
     untracked: true
     expire_in: '1 week'