[test] Mark FFI fake HTTP test as slow

Fixes #61583

Change-Id: Ia91c28692fd0b07af0674db654b0ee2d9409fbe9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/452980
Commit-Queue: Liam Appelbe <[email protected]>
Reviewed-by: Nate Biggs <[email protected]>

Author: liamappelbe

samples/samples.status

@@ -2,6 +2,8 @@
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
+ffi/http/test/http_test: Pass, Slow # https://dartbug.com/61583
+
 [ $sanitizer == asan ]
 ffi/samples_test: SkipByDesign # FFI skips, see ffi.status