collectd: improve test

Even if collectd had broken linkage, the old test would pass.

Closes #15610.

Signed-off-by: ilovezfs <ilovezfs@icloud.com>

Formula/collectd.rb

@@ -88,9 +88,19 @@ class Collectd < Formula
   end
 
   test do
+    log = testpath/"collectd.log"
+    (testpath/"collectd.conf").write <<-EOS.undent
+      LoadPlugin logfile
+      <Plugin logfile>
+        File "#{log}"
+      </Plugin>
+      LoadPlugin memory
+    EOS
     begin
-      pid = fork { exec sbin/"collectd", "-f" }
-      assert shell_output("nc -u -w 2 127.0.0.1 25826", 0)
+      pid = fork { exec sbin/"collectd", "-f", "-C", "collectd.conf" }
+      sleep 1
+      assert_predicate log, :exist?, "Failed to create log file"
+      assert_match "plugin \"memory\" successfully loaded.", log.read
     ensure
       Process.kill("SIGINT", pid)
       Process.wait(pid)