cmd/go: make get_dash_t test more specific

cmd/go: make get_dash_t test more specific

Currently it uses "go list ...", which includes all packages in
the known universe, and may include unresolved dependencies. The
test for issue #8181 is specifically for that the test dependency
of package b is downloaded. Test that specifically.

Change-Id: Icfbd7e197698b10ae4bc7c8aa3b0f2c477ca6b8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/477837
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>