UBI: make tests modes dynamic

Similarly to the debugging checks and message, make the test modes
be dynamically selected via the "debug_tsts" module parameter or
via the "/sys/module/ubi/parameters/debug_tsts" sysfs file. This
is consistent with UBIFS as well.

And now, since all the Kconfig knobs became dynamic, we can remove
the Kconfig.debug file completely.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>