Add sscanf %[ tests and fix the bug that fell out.

Strictly, POSIX says "If a '-' is in the scanlist and is not the first
wide character, nor the second where the first wide character is a '^',
nor the last wide character, the behavior is implementation-defined",
but it seems unreasonable for swscanf to interpret `a-c` differently
from sscanf. Make ours behave the same as each other by making swscanf
work the same as sscanf.

Bug: http://b/68672236
Test: ran tests
Change-Id: Ia84805897628d7128e901b468e02504373730e61