openpgp: split up tests and keys

keys_test.go was slowing down my editor because it was getting too
large. It helps to remove the keys of the file as they contain extremely
long lines and large strings.

Change-Id: I8d193179ddc32438b7233f0f9ca8c57c928a0436
Reviewed-on: https://go-review.googlesource.com/c/138997
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>