tstest/tlstest: simplify, don't even bake in any keys

I earlier thought this saved a second of CPU even on a fast machine,
but I think when I was previously measuring, I still had a 4096 bit
RSA key being generated in the code I was measuring.

Measuring again for this, it's plenty fast.

Prep for using this package more, for derp, etc.

Updates #16315

Change-Id: I4c9008efa9aa88a3d65409d6ffd7b3807f4d75e9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/tlstest/tlstest_test.go

@@ -0,0 +1,21 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package tlstest
+
+import (
+	"testing"
+)
+
+func TestPrivateKey(t *testing.T) {
+	a := privateKey("a.tstest")
+	a2 := privateKey("a.tstest")
+	b := privateKey("b.tstest")
+
+	if string(a) != string(a2) {
+		t.Errorf("a and a2 should be equal")
+	}
+	if string(a) == string(b) {
+		t.Errorf("a and b should not be equal")
+	}
+}