Allow cancelling of a test

[SimonRichardson]

We're spawning new go tests, but if you've got a test that's hanging/deadlocked, then it would be nice to kill the spawned job. The workaround is to kill the parent process (vscode), which isn't ideal.

[babakks]

Thanks for the suggestion, @SimonRichardson. The new version of the extension (v0.0.15) supports the cancelling of test runs. To cancel a test, you'd need to click on the stop button next to the running test's name in the "Test Output" pane. That will kill the underlying go test ... process.