`head` name is a bit ambiguous because it might be perceived as
referring to currently latest transaction ID, but in that test it
refers to latest transaction ID when the test starts.

Rename it to at0 for clarity.

Just added test_zodbcommit_cmd already uses at0  to represent initial
transaction ID (and at1 + at2 to represent follow-up transaction IDs).

In general using atX naming comes from WCFS functional tests where it is
used widely - see e.g.
https://lab.nexedi.com/nexedi/wendelin.core/-/blob/07087ec8/wcfs/wcfs_test.py#L1539-1581