Add an example slow endpoint to help with testing

How other things handle slow responses.

guix-data-service/web/controller.scm

@@ -874,6 +874,21 @@
      (('GET "compare-by-datetime" _ ...) (delegate-to compare-controller))
      (('GET "jobs" _ ...)   (delegate-to jobs-controller))
      (('GET "job" job-id)   (delegate-to jobs-controller))
+     ;; Example slow endpoint to help with testing
+     (('GET "slow")
+      (list (build-response
+             #:code 200
+             #:headers '((content-type . (text/plain
+                                          (charset . "utf-8")))
+                         (vary . (accept))))
+            (lambda (port)
+              (force-output port)
+              (for-each
+               (lambda (i)
+                 (put-string port (simple-format #f "slow ~A\n" i))
+                 (force-output port)
+                 (sleep 5))
+               (iota 12)))))
      (('GET _ ...) (delegate-to nar-controller))
      ((method path ...)
       (render-html