ci: twisterlib: Modify the overlay regular expression

Modify the regex used to merge overlays specified for
test samples in twister tests.
Overlays without any prefixes will be merged and passed to cmake.
Overlays with prefixes won't be merged but passed to cmake as they are.

Signed-off-by: Piotr Węgliński <piotr.weglinski@nordicsemi.no>