encoding/xml: add more EncodeToken tests.

There are no behavior changes in this CL, only specifying the status
quo. A follow-up CL, https://go-review.googlesource.com/#/c/2660/, will
change the EncodeToken behavior.

Change-Id: I6ecbcfb05ae681de71fa1099d054df2826ed4acb
Reviewed-on: https://go-review.googlesource.com/4167Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>