crypto: test for unexpected concrete methods in interface value returns

crypto: test for unexpected concrete methods in interface value returns

Change-Id: I24188ad5f51953b2fbdef7487acc4ab6b1d77575
Reviewed-on: https://go-review.googlesource.com/c/go/+/638175
Auto-Submit: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>