oauth2: deflake test relying on time.Now values

It was particularly flaky on Windows where time.Now doesn't advance as
fast as elsewhere.

Change-Id: Ic0b7c3e4c69389009d1e28750be0cd203fa770aa
Reviewed-on: https://go-review.googlesource.com/c/157578
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

token.go

@@ -118,13 +118,16 @@ func (t *Token) Extra(key string) interface{} {
 	return v
 }
 
+// timeNow is time.Now but pulled out as a variable for tests.
+var timeNow = time.Now
+
 // expired reports whether the token is expired.
 // t must be non-nil.
 func (t *Token) expired() bool {
 	if t.Expiry.IsZero() {
 		return false
 	}
-	return t.Expiry.Round(0).Add(-expiryDelta).Before(time.Now())
+	return t.Expiry.Round(0).Add(-expiryDelta).Before(timeNow())
 }
 
 // Valid reports whether t is non-nil, has an AccessToken, and is not expired.

token_test.go

@@ -34,13 +34,17 @@ func TestTokenExtra(t *testing.T) {
 
 func TestTokenExpiry(t *testing.T) {
 	now := time.Now()
+	timeNow = func() time.Time { return now }
+	defer func() { timeNow = time.Now }()
+
 	cases := []struct {
 		name string
 		tok  *Token
 		want bool
 	}{
 		{name: "12 seconds", tok: &Token{Expiry: now.Add(12 * time.Second)}, want: false},
-		{name: "10 seconds", tok: &Token{Expiry: now.Add(expiryDelta)}, want: true},
+		{name: "10 seconds", tok: &Token{Expiry: now.Add(expiryDelta)}, want: false},
+		{name: "10 seconds-1ns", tok: &Token{Expiry: now.Add(expiryDelta - 1*time.Nanosecond)}, want: true},
 		{name: "-1 hour", tok: &Token{Expiry: now.Add(-1 * time.Hour)}, want: true},
 	}
 	for _, tc := range cases {