os/signal: disable loading of history during test

os/signal: disable loading of history during test

    Bring in https://golang.org/cl/98616 from gc tip.

    Original CL description:

        This change modifies Go to disable loading of users' shell history for
        TestTerminalSignal tests. TestTerminalSignal, as part of its workload,
        will execute a new interactive bash shell. Bash will attempt to load the
        user's history from the file pointed to by the HISTFILE environment
        variable. For users with large histories that may take up to several
        seconds, pushing the whole test past the 5 second timeout and causing
        it to fail.

    Reviewed-on: https://go-review.googlesource.com/107624

From-SVN: r259452