I've run into this before as well. Recursive calls to "go build" from tests use the go from your path, not the go that was used to run "go test". There are two workarounds: use all.bash, or set up GOROOT/PATH with the directory you're running in.

I don't see any obvious fix. All these tests call testenv.MustHaveGoBuild(t) or variants, maybe we could have them return the right path to use?

There's already this in testenv:

// GoToolPath reports the path to the Go tool.
// If the tool is unavailable GoToolPath calls t.Skip.
// If the tool should be available and isn't, GoToolPath calls t.Fatal.
func GoToolPath(t *testing.T) string {
        MustHaveGoBuild(t)

There's probably stuff not using it.

Excellent, that's new to me. We should use GoToolPath instead of the MustHaveGoBuild + "go" combo.
Same for MustHaveGoRun, I guess.

CL https://golang.org/cl/28089 mentions this issue.