crypto: use testenv.Executable(t) instead of os.Args[0] in tests

crypto: use testenv.Executable(t) instead of os.Args[0] in tests

Change-Id: Ib0ec1f05e51a4295a9369d6e8c6b61976a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/735260
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>