twister: bugfix: Make elf scan more precise.

When scanning for available elfs, twister discards ones with `_pre`
in their name, to not include prebuilds like 'zephyr_pre0.elf`.
However, the scan looks at the whole test name. If _pre is in the name
(e.g. _precision) it would be wrongly discarded ending with twister
reporting error that no elfs were found. This commit makes the discard
to look only at the last part of the name (i.e. not including test name
but only a lowest level, where zephyr.elf etc are located.)

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>