Due to forgotten ... it was not passing prepared argv as argv to
fmt.Errorf, but as a single slice argument. The result was messy, for
example

    persistent_test.go:176: deactivate refcnt < 0: panic error unexpected:
        have: "[t.zodb.MyObject 000000000000000c](%!s(MISSING)): deactivate: refcnt < 0"
        want: "t.zodb.MyObject(000000000000000c): deactivate: refcnt < 0"

Using the occasion add details to deactivate refcnt panic, which
made me to discover this bug.