tests: vnstat: Avoid call to 'getservbyname'.

This would break "make as-derivation" because /etc/services is
unavailable in the build environment:

  [ 38/ 80] loading...	 95.0% of 40 filesBacktrace:
  In guix/build/compile.scm:
      249:8 19 (compile-files "." "/gnu/store/s5nadqd6hkzivkxp33svwqslfn608ng5-guix-system-tests" ("gnu/tests/audio.scm" "gnu/tests/base.scm" "gnu/tests/ci.scm" "gnu/tests/cups.scm" "gnu/tests/d…" …) …)
  […]
  In unknown file:
	     0 (getserv "discard" "tcp")

  ERROR: In procedure getserv:
  In procedure getserv: no such service discard
  builder for `/gnu/store/…-guix-system-tests.drv' failed with exit code 1

* gnu/tests/vnstat.scm (run-vnstat-test): Hard-code 'guest-port' instead
of calling 'getservbyname'.

1 files changed, 3 insertions, 6 deletions

diff --git a/gnu/tests/vnstat.scm b/gnu/tests/vnstat.scm
index d1be98e342..c498b987ff 100644
--- a/gnu/tests/vnstat.scm
+++ b/gnu/tests/vnstat.scm
@@ -61,12 +61,9 @@
   (define forwarded-port 9999)
 
   (define vm
-    (let* ((inetd-service-name "discard")
-           (inetd-service-proto
-            (inetd-entry-protocol inetd-service-entry-config))
-           (guest-port
-            (servent:port (getservbyname inetd-service-name
-                                         inetd-service-proto))))
+    ;; The 'discard' port is 9.  Avoid using 'getservbyname' as that might
+    ;; fail depending on what /etc/services has (if it's available).
+    (let ((guest-port 9))
       (virtual-machine
        (operating-system os)
        (port-forwardings `((,forwarded-port . ,guest-port))))))