controlclient: set default TimeNow function

Panic on call to nil func found by expanded ipn/e2e_test.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

control/controlclient/auto.go

@@ -147,6 +147,9 @@ func NewNoStart(opts Options) (*Client, error) {
 	if opts.Logf == nil {
 		opts.Logf = func(fmt string, args ...interface{}) {}
 	}
+	if opts.TimeNow == nil {
+		opts.TimeNow = time.Now
+	}
 	c := &Client{
 		direct:   direct,
 		timeNow:  opts.TimeNow,