harden configloading system test

harden configloading system test

The configloading system script attempts multiple `rndc
{reconfig,reload}` commands without ensuring the system left
exclusive mode; which normally raise an RNDC error as the server is
currently reloading already. This used to work because the request was
enqueued while the server was in exclusive mode, and was processed
after the server `reload_status` was reset to `NAMED_RELOAD_DONE`.

Due to the fact the exclusive mode is not retaken after
`apply_configuration()` by `load_zones()`, the scheduling of
pending tasks is changed and, regularly, the RNDC command sent by the
test is processed before `NAMED_RELOAD_DONE` is set. This is the same
kind of issue the views system tests had, solved by
`4b2dcb3128fbd5af4609a5a73aeeee1f93bde237`

Fix the problem by waiting for a log line matching the end of
the reloading phase.