x/build/cmd/coordinator: some subrepos want to run tests in both module and GOPATH modes

[dmitshur]

coordinator already had support for running tests (at least trybots) in module mode, it's used for oauth2 and build subrepos:

https://github.com/golang/build/blob/34b995b40b0d31f2760a2f57b9ce74e17925702c/cmd/coordinator/coordinator.go#L2506-L2507

Some subrepos, such as x/tools, may want to soon start running their tests in both GO111MODULE=on and GO111MODULE=off modes, in order to make sure there are no regressions in either mode.

We should add support for this to the coordinator.

/cc @bradfitz @matloob

[dmitshur]

I see two ways this can be implemented:

• a more lightweight way (i.e., without having to run all tests twice) by adding a new GOPATH mode builder for just one GOOS/GOARCH, similar to how we do that for running tests on previous supported versions of Go, e.g., see here
• a more heavyweight way, run tests in both modes on each GOOS/GOARCH combination

[gopherbot]

Change https://golang.org/cl/194277 mentions this issue: cmd/coordinator: detect current build mode in subrepo tests

[dmitshur]

It turned out we had sufficient coverage so none of the subrepos needed this,
and we got by without ever implementing it. So the decision here is "won't fix".

Closing in favor of #48875.