all: use tstest.Replace more

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

tstest/tstest.go

@@ -15,10 +15,11 @@ import (
 
 // Replace replaces the value of target with val.
 // The old value is restored when the test ends.
-func Replace[T any](t *testing.T, target *T, val T) {
+func Replace[T any](t testing.TB, target *T, val T) {
 	t.Helper()
 	if target == nil {
 		t.Fatalf("Replace: nil pointer")
+		panic("unreachable") // pacify staticcheck
 	}
 	old := *target
 	t.Cleanup(func() {