cmd/go: TestNewReleaseRebuildsStalePackagesInGOPATH modifies code in GOROOT

[bcmills]

go test cmd/go should succeed even if the user cannot write to GOROOT, as may be the case if the Go toolchain is provided by a system distribution. (See also #28387.)

Unfortunately, today TestNewReleaseRebuildsStalePackagesInGOPATH assumes that it can write to GOROOT/src:

go/src/cmd/go/go_test.go

Line 903 in dbd323b

sys := runtime.GOROOT() + "/src/runtime/internal/sys/sys.go"

That not only risks a failed write to an unwritable directory, but also makes go test cmd/go dangerous to run concurrently with any build information, and potentially introduces spurious diffs during testing in development copies of the go repository.

[bcmills]

One possible resolution might be to copy relevant parts of GOROOT into a temporary directory.

[bcmills]

CC @ianlancetaylor @rsc

[ianlancetaylor]

I sent a fix for that test but there are probably others.

[gopherbot]

Change https://golang.org/cl/154460 mentions this issue: cmd/go: don't modify GOROOT in TestNewReleaseRebuildsStalePackagesInGOPATH