go/build: make TestDependencies work again

CL 243940 accidentally broke TestDependencies such that it always passed.
Make it work again, and add a test so that it won't break in the same way.

This revealed that the new embed package was missing from TestDepencies,
so add it.

Fixes #43249

Change-Id: I02b3e38dd35ad88880c4344d46de13b7639aa4c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/279073


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>