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.

Change-Id: I11b2f83ee91f51fa1e9774a39181ab365f9a6b3a
GitHub-Last-Rev: 7efdf616a2fcecdf479420fc0004057cee2ea6b2
GitHub-Pull-Request: golang/go#24255
Reviewed-on: https://go-review.googlesource.com/98616Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>