[release-branch.go1.12] cmd/vendor: remove _test.go and testdata files

Binary files included in testdata directories can cause Apple’s
notarization service to reject us since they don’t abide by their
strict requirements.

To emulate go mod vendor, remove all _test.go and testdata files
from the vendor directory and update the instructions.

Updates #34986
Fixes #35747

Change-Id: I5cde905fc78838d2e3b1519dab4aeee13d8d5356
Reviewed-on: https://go-review.googlesource.com/c/go/+/208227


Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>