cmd/go: convert TestIssue7108 to the script framework

Part of converting all tests to script framework to improve
test parallelism.

Updates #36320
Updates #17751

Change-Id: Ic16a0c6c1eabeba003b9cff620e2911078fd6cf8
Reviewed-on: https://go-review.googlesource.com/c/go/+/212813


Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>