tstest: prepare for Clock API changes

This change introduces tstime.NewClock and tstime.ClockOpts as a new way
to construct tstime.Clock. This is a subset of #8464 as a stepping stone
so that we can update our internal code to use the new API before making
the second round of changes.

Updates #8463

Change-Id: Ib26edb60e5355802aeca83ed60e4fdf806c90e27
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>

net/dns/recursive/recursive_test.go

@@ -36,9 +36,9 @@ func init() {
 }
 
 func newResolver(tb testing.TB) *Resolver {
-	clock := &tstest.Clock{
+	clock := tstest.NewClock(tstest.ClockOpts{
 		Step: 50 * time.Millisecond,
-	}
+	})
 	return &Resolver{
 		Logf:    tb.Logf,
 		timeNow: clock.Now,