Try removing isolate.kill()

At one point isolates were killed when the loader was closed as the
runner is shutting down. Later the kill was moved to when the test suite
is done. There was no specific comment about the `isolate.kill()`
changes. https://codereview.chromium.org//1206033004

Before that the `isolate.kill` on close goes back to the original
implementation of the loader.
https://codereview.chromium.org//920703006

No internal tests seem to be relying on this behavior, and nothing times
out when it is removed.

Check whether there are any tests on CI that validate this behavior.

pkgs/test_core/lib/src/runner/vm/platform.dart

@@ -80,7 +80,6 @@ class VMPlatform extends PlatformPlugin {
         rethrow;
       }
       outerChannel = MultiChannel(IsolateChannel.connectReceive(receivePort));
-      cleanupCallbacks.add(isolate.kill);
     }
     cleanupCallbacks.add(outerChannel.sink.close);