Append #testem in test_page URL automatically

[sideroad]

[stvnmntjy]

Despite still being open, this seems to be the default. But an earlier testem commit has (kinda) deprecated it.

If users need to hash, they still can in the testem config file (or you could support adding it in the task).

For those not needing hashing, tests won't run with the auto-append.