oauth2: fix expiration time window check

Before CL 68017, the test verified that tokens expire exactly one day
after they are issued, within the time that Exchange() takes to execute.
A refactoring in the CL extended this time window to a whole day, making
the check much more lenient.

See: https://go-review.googlesource.com/c/oauth2/+/168017
Change-Id: I479db429f64b25a711624817c13f068b4675163e
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/630495
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Junyang Shao <[email protected]>
Reviewed-by: Sean Liao <[email protected]>
Auto-Submit: Sean Liao <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: ccmtaylor

oauth2_test.go

@@ -300,7 +300,7 @@ func testExchangeRequest_JSONResponse_expiry(t *testing.T, exp string, want, nul
 	conf := newConf(ts.URL)
 	t1 := time.Now().Add(day)
 	tok, err := conf.Exchange(context.Background(), "exchange-code")
-	t2 := t1.Add(day)
+	t2 := time.Now().Add(day)
 
 	if got := (err == nil); got != want {
 		if want {