Mock driver config uses `time.Duration` fields but we initialize them
inconsistently, as time.Duration sometimes and as duration strings other
times. Previously, `mapstructure` handles it and does the right thing.
This is no longer the case with MsgPack. I could not find a good way to
bring back old behavior without too much complexity. `MsgPack` extended
types weren't ideal here as we lose type information (e.g. int64 vs
string), and the input is a generic map and not a MsgPack serialization
of duration.
As such, I went with the simple solution of declaring the config field
as duration string, and panicing if the test doesn't pass a valid
string.
I found this to cause the smallest change in tests, but we can
alternatively force all to be int64 instead.
