tests: Fix tailon test.

This is a follow-up of 73eeeeafbb.

* gnu/tests/web.scm (run-tailon-test): Wait for the TCP port 8080 instead of
relying on start-service.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>

gnu/tests/web.scm

@@ -481,11 +481,7 @@ HTTP-PORT."
           (test-begin "tailon")
 
           (test-assert "service running"
-            (marionette-eval
-             '(begin
-                (use-modules (gnu services herd))
-                (start-service 'tailon))
-             marionette))
+            (wait-for-tcp-port 8080 marionette))
 
           (test-equal "http-get"
             200