test: compile source files as if from "test" module

test: compile source files as if from "test" module

This CL updates test/run.go to compile xxx.dir/x.go with a package
path of "test/x" instead of just "x". This prevents collisions with
standard library packages.

It also requires updating a handful of tests to account for the
updated package paths.

Fixes #25693.

Change-Id: I49208c56ab3cb229ed667d547cd6e004d2175fcf
Reviewed-on: https://go-review.googlesource.com/c/go/+/395258
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>