tests: inetd: Avoid race condition when looking for PID file.

* gnu/tests/networking.scm (run-inetd-test): Use ‘wait-for-file’ instead
of ‘file-exists?’.

Change-Id: I3e596814485428fb81595d81c4e03519340c6576

gnu/tests/networking.scm

@@ -349,9 +349,8 @@ port 7, and a dict service on port 2628."
 
           ;; Make sure the PID file is created.
           (test-assert "PID file"
-            (marionette-eval
-             '(file-exists? "/var/run/inetd.pid")
-             marionette))
+            (wait-for-file "/var/run/inetd.pid" marionette
+                           #:timeout 30))
 
           ;; Test the echo service.
           (test-equal "echo response"