cmd/go: make TestScript/mod_get_patchmod self-contained

I find it pretty difficult to reason about test-dependency modules
when they aren't in the same file as the rest of the test.

Now that 'go get' supports replacements (CL 258220 and CL 266018),
we can localize tests that need 'go get' but don't specifically depend
on module proxy semantics.

For #36460
For #37438

Change-Id: Ib37a6c170f251435399dfc23e60d96681a81eadc
Reviewed-on: https://go-review.googlesource.com/c/go/+/266369


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>