tests: nffs_fs_api: Add performance test for nffs

This adds performance test for nffs with open, close, read, write and
unlink sub test cases.

Origin: Original

Signed-off-by: Grzegorz Kolodziejczyk <grzegorz.kolodziejczyk@codecoup.pl>
Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@codecoup.pl>