go: encoding/gob: test frequently failing with "signal: killed" in longtest builder

The encoding/gob test is frequently failing in the longtest builder with output like:

signal: killed
FAIL	encoding/gob	13.202s

(That’s the entire output.)

First example I saw was in https://build.golang.org/log/608043bae4b6ccc72e79f266fd92fa51410517b1, but the failure mode doesn’t obviously relate to the associated CL (https://golang.org/cl/172418).

I haven’t been able to replicate the failure locally so far. I don’t know whether the failure is due to recent changes in the compiler (@mdempsky, @cuonglm) or in cmd/go (myself and @jayconrod).

The transition from passing to failure was partially masked by #31263.