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>