Fail isolate suites when the test isolate exits

[natebosch]

When a VM test suite runs in an isolate, nothing noticed if the isolate
exited. The isolate channel does not close when the isolate exits, so:

• If the isolate exited while the suite was loading, loading waited
  forever for the serialized suite. For example, a test file whose
  main calls Isolate.exit() hung the run.
• If the isolate exited while a test was running, that test never
  completed. The run hung, and interrupting it hung again while it
  waited for the test to finish.

Spawn test isolates with an onExit port. When the isolate exits,
close the port the isolate channel receives from. The channel then ends
once it has delivered any messages the isolate sent before exiting:

• An error the isolate reported while loading is still what fails the
  load.
• Otherwise loading fails with "Connection closed before test suite
  loaded.", as it already does for exe and cli suites when the
  socket closes.
• Tests that haven't finished are reported as not completed.

Closing the port doesn't end the channel if the isolate hasn't
connected yet, so waiting for the channel ID also races the exit
message.

Only listen for data on the exit port, since it is also closed as part
of the suite's normal cleanup.

[github-actions]

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