On the Mac port, this should work by remotely inspecting the DRT/WKTR's inspector instance. Nothing needs to be done for that to work; what would help is adding a "--wait" flag or something to the test harness that will pause until a remote inspector frontend attaches. Otherwise, the test will complete before you have a chance to inspect. It should also override the slow test timeout, since that is trivially triggered when staring at the debugger (the timer includes paused times).
<rdar://problem/21847843>