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>

src/crypto/rsa/rsa_test.go

@@ -145,6 +145,12 @@ d8Y7
 }
 
 func testKeyBasics(t *testing.T, priv *PrivateKey) {
+	defer func() {
+		if t.Failed() {
+			t.Logf("failed key: %#v", priv)
+		}
+	}()
+
 	if err := priv.Validate(); err != nil {
 		t.Errorf("Validate() failed: %s", err)
 	}