oauth2_test: regression in expiration time check

[ccmtaylor]

Before CL 168017, 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.

Fixed in https://go-review.googlesource.com/c/oauth2/+/630495