[vm/gardening] Mark get_vm_timeline_rpc_test as slow.

Threads appear to still be doing real work when the timeout occurs. Local testing confirms
that the test is actually slow.

Change-Id: I3b0dfa467c5b2eeda481e31716cfc1e79583100d
Reviewed-on: https://dart-review.googlesource.com/c/83762
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Auto-Submit: Samir Jindel <sjindel@google.com>

runtime/observatory/tests/service/service_kernel.status

@@ -136,7 +136,7 @@ get_object_rpc_test: Pass, RuntimeError # Please triage / issue #27806
 
 [ $arch == ia32 && $builder_tag == optimization_counter_threshold ]
 eval_regression_flutter20255_test: SkipSlow # Timeout
-get_vm_timeline_rpc_test: Pass, Timeout # Issue 35022
+get_vm_timeline_rpc_test: Slow
 rewind_optimized_out_test: SkipSlow # Timeout
 
 [ $arch != simarm && $arch != simarm64 && $arch != simdbc && $compiler == dartk ]