allow args.background parameter for go.test.* commands

[leep-frog]

Fixes #3154

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[leep-frog]

I just filled out the CLA. Doesn't look like I have permission to re-run checks though :(

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Patch Set 1:

(1 comment)

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[gopherbot]

This PR (HEAD: 38b4cbb) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/vscode-go/+/567995.

Important tips:

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