tstest: add method to Replace values for tests

We have many function pointers that we replace for the duration of test and
restore it on test completion, add method to do that.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

tstest/tstest_test.go

@@ -0,0 +1,24 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package tstest
+
+import "testing"
+
+func TestReplace(t *testing.T) {
+	before := "before"
+	done := false
+	t.Run("replace", func(t *testing.T) {
+		Replace(t, &before, "after")
+		if before != "after" {
+			t.Errorf("before = %q; want %q", before, "after")
+		}
+		done = true
+	})
+	if !done {
+		t.Fatal("subtest didn't run")
+	}
+	if before != "before" {
+		t.Errorf("before = %q; want %q", before, "before")
+	}
+}