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/dnscache/messagecache_test.go

@@ -18,9 +18,9 @@ import (
 )
 
 func TestMessageCache(t *testing.T) {
-	clock := &tstest.Clock{
+	clock := tstest.NewClock(tstest.ClockOpts{
 		Start: time.Date(1987, 11, 1, 0, 0, 0, 0, time.UTC),
-	}
+	})
 	mc := &MessageCache{Clock: clock.Now}
 	mc.SetMaxCacheSize(2)
 	clock.Advance(time.Second)