The common construct seen in the BIND 9 source is func(isc_mem_t *mctx, ...).
Unfortunately, the dnstest.{h,c} has been using mctx as a global symbol, which
in turn generated a lot of errors when update.c got included in update_test.c.

As a rule of thumb, we should avoid naming global symbols with generic names
(like mctx) and we should prefix them with "namespace" (like dt_mctx).