crypto/rsa: log key on test failure

crypto/rsa: log key on test failure

For #74326

Change-Id: If1e61db22c9e7192e5dd56cd36141e5b6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/734640
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>