When -test.fullpath flag is provided to go test,
go test displays the full file names in error messages.

Fixes #37708

Change-Id: I6096e75ed816fd42205810f20624e495047a73a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/463837


Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>