tests: fs: fcb: Make fixed endmarker test optional

The commit splits default FCB scenario into one with endmarker
enabled, the original one, and one with disabled.
This allows to test the FCB in cases when CONFIG_FCB_ALLOW_FIXED_ENDMARKER
is set and not set.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>