Table_open_cache gets adjusted on server startup to prevent an out of
file descriptor error. However this means that when we reset its value
to default, it does not get re-adjusted. This leads to the mtr
consistency check to fail with different server status at the end of the
test case as opposed to when it started.

To fix the problem, do not make use of the DEFAULT keyword to set the
variable back, instead save the value before any modifications and
restore it from there.