x/build: support x/playground tests that need additional dependencies

[dmitshur]

Right now the x/playground repo is tested via the usual go test ./... as other x/ repos.
As described in #24823, there are tests that need additional dependencies to run (Docker).
This is the tracking issue for that.

CC @golang/tools-team.

[findleyr]

As described in #69403, while I think we should support Dockerized tests for some projects such as vscode-go or pkgsite, my intuition is that an the playground relies on Docker more than it really needs to (it's just running the go command, after all!). We should probably address this issue by removing Docker from the development and test flow in x/playground.

[dmitshur]

Agreed – if achieving x/playground's testing needs can be made to work without the Docker dependency, that path would be great. Recently @hyangah is exploring a similar direction for vscode-go's tests, tracked in golang/vscode-go#3533.