cmd/link/internal/ld: split varDeclCoords tests

cmd/link/internal/ld: split varDeclCoords tests

This test set is a bit hard to follow due to trying to test both
variable and function declaration location information.

Now that we have additional helpers to avoid duplication, it isn't too
much work to split them up into individually more understandable tests.

Change-Id: I619ac82ac3b5d00683e22a4a2064e2a5b15e8ce9
Reviewed-on: https://go-review.googlesource.com/c/go/+/458197
Auto-Submit: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>