fstests: add helper _require_scratch_feature

fstests: add helper _require_scratch_feature

The helper is used to test if a specific filesystem feature can
be enabled. Currently only implemented testing overlayfs features.

Overalyfs features (e.g. redirect_dir, index) are configurable from
Kconfig (the build default), by module parameter (the system default)
and per mount using the mount option ${feature}=[on|off].

The helper is going to be used by overlay tests, which depend
on the inodes index feature.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>