testing: Avoid concurrent go test invocations when "Test on Save" is enabled

[CAFxX]

Is your feature request related to a problem? Please describe.
Some projects have tests that can not be run in parallel as they may require external resources (e.g. a listener port, or an external database, or a file on disk, or some other state/config in a second process)

When I enable "test on save" and save again quickly before the previous go test completes (that was automatically started when I saved previously), a new go test is invoked and this will often cause both the previous and the current invocations to (eventually) fail.

Describe the solution you'd like
Either stop the previous go test run unconditionally before starting a new one, or give it as an option (e.g. by default maintain the current behavior that allow parallel go test invocations, and allow user to configure vscode-go to kill the previous run when starting the new one).

Describe alternatives you've considered
Another alternative would be waiting for the previous run to complete. But if I just saved again is because most likely I noticed something wrong in the code, and therefore I probably don't care about the results of the previous run. Note that this applies even if multiple go test invocations actually were able to run concurrently without issue.

Another option would be to never make mistakes. Sometimes I wish I could do that.

Additional context

[gopherbot]

Change https://golang.org/cl/291969 mentions this issue: package.json: add disableConcurrentTests setting