scripts: Add invariant log closing

If BB tests failed, crashed, etc., log handles
could linger and crash test cleanup.
Adding log closing to a `finally` section
should prevent those issues.

Loggers should not be set up as globals,
as it makes testing much harder.
Running multiple Twisters may cause for the
logfiles to be still "in use".

When exiting main, close all logfiles
and remove their handlers from all loggers.
Do that for conftest as well.

Signed-off-by: Lukasz Mrugala <lukaszx.mrugala@intel.com>