This behavior and test was introduced in 0ae3d4edc9.
It is not consistent with the other test introduced in the same commit,
where an incorrectly typed access_token does produce an error. Since a
*Token with a blank AccessToken is invalid, it is allowing an invalid
token to be returned without error.
Cleans up some tests responding with invalid data.
Change-Id: I777eb7a82ef598dc9042542ae65f8dce6768902e
Reviewed-on: https://go-review.googlesource.com/85659
Reviewed-by: Andrew Bonventre <andybons@golang.org>