cmd/go/internal/mvs: test a downgrade where the target explicitly requires itself

cmd/go/internal/mvs: test a downgrade where the target explicitly requires itself

Also clean up the test assertions, and add a check for assertions
missing function invocations (there was one).

For #37438

Change-Id: Iafbfeae2c25217eac894181e01480b25b7cffbd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/266859
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>