all: eliminate vet diagnostics

For golang/go#74011

Change-Id: I189c5aba554a578bee1fd351edc30cd5cf4d0ed6
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/714960
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: David Chase <drchase@google.com>

otr/otr_test.go

@@ -402,7 +402,7 @@ func TestAgainstLibOTR(t *testing.T) {
 	// This test requires otr.c.test to be built as /tmp/a.out.
 	// If enabled, this tests runs forever performing OTR handshakes in a
 	// loop.
-	return
+	t.Skip("This test requires otr.c.test to be built as /tmp/a.out")
 
 	alicePrivateKey, _ := hex.DecodeString(alicePrivateKeyHex)
 	var alice Conversation