Fix error reporting in process-async-https-with-delay

* test/src/process-tests.el (process-async-https-with-delay): Use
'plist-get' instead of 'assq' in testing for a connection error.
The 'status' variable is a plist, not an alist.  (Bug#55849)

test/src/process-tests.el

@@ -939,7 +939,7 @@ Return nil if FILENAME doesn't exist."
                         (< (float-time) (+ t0 limit)))
               (sit-for 0.1)))
           (should status)
-          (should-not (assq :error status))
+          (should-not (plist-get status ':error))
           (should buf)
           (should (> (buffer-size buf) 0))
           )