cmd/go: do not check for a built binary in TestScript/mod_get_fossil
This test hasn't passed since CL 349997, but the failure was not
detected because the Go project's builders do not have a 'fossil'
binary installed (#48802).
For #43684
Change-Id: I25544574ab48f4f146ae3795e541179e78815758
Reviewed-on: https://go-review.googlesource.com/c/go/+/354149
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com> Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>