[release-branch.go1.12] cmd/vet: add tests for point-release issues

[release-branch.go1.12] cmd/vet: add tests for point-release issues

Add explicit tests for:

 #30465	cmd/vet: Consider reverting tag conflict for embedded fields
 #30399	cmd/vet: possible to get a printf false positive with big.Int

because we have managed not to fix them in the last
couple point releases, and it will be too embarrassing
to do that yet again.

Change-Id: Ib1da5df870348b6eb9bfc8a87c507ecc6d44b8dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/174520
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>