tests: ztest: fixed off-by-one in sys_bitfield_find_first_clear

Calling __builtin_ffsl(neg_bitmap) returns first bit set in the word,
e.g. 4 if bitmap is 11111000. As this must translate to zero-based index
3, one must be subtracted from the result.

Signed-off-by: Morten Priess <mtpr@oticon.com>
1 file changed