scripts: west_commands: build: Fix warning during rebuild

5718af02 introduced a regression where a warning was printed
about unknown BOARD when rebuilding. Commit moved _find_board()
to an earlier stage and that was failing during the rebuild. Instead
of moving board finding earlier we can move parsing of the test_item
to the later stage as it is needed only just before running cmake.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>