x/build/cmd/golangbuild: run subrepo tests from outside their repositories

[bcmills]

(This is essentially the golang.org/x/[…] version of #30316.)

go.mod files in testdata directories cut off the directory from the rest of the module (see #27852). As a result, the corresponding tests are likely to fail.

Unfortunately, at least one of the golang.org/x repos already includes such files: I am aware of some in golang.org/x/tools/go/packages/testdata (CC @heschik, @matloob), and there may be others as well.

Rather than expecting all Go developers to remember to avoid adding extraneous go.mod files, we should configure builders and TryBots to run the tests for golang.org/x modules outside the module, either in addition to or instead of running them from within a cloned repository.

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(The simplest alternative I am aware of is to construct a symlink overlay within a temporary directory, as is done in various misc/cgo tests.)

CC @dmitshur @toothrot @bradfitz

[gopherbot]

Change https://golang.org/cl/196258 mentions this issue: go/packages: remove go.mod files

[dmitshur]

/cc @ianthehat FYI. This is the issue I mentioned to you in person just now.