sys: linear_range: fix index calculation when spanning across groups

When dealing with groups, the specified window in
linear_range_group_get_win_index can span across multiple groups. In the
current implementation, if maximum value was not in the same sub-window
as the minimum value the function would return -EINVAL. This patch fixes
the problem and updates test cases to cover such scenario.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>