tests: Add generating metadata using mfssetgoal N+/-

This commit introduces in generating metadata of increasing or decreasing
file's goal in metadata tests. During the development, a bug in mfssetgoal
documentation was found and fixed.

Change-Id: Ia6411e670384413e5eaa6d5888ba378c0a8903cc
tests: Add generating metadata using mfssettrashtime N+/-

This commit introduces in generating metadata of increasing or decreasing
file's trashtime in metadata tests.  During the development, a bug in
mfssettrashtime documentation was found and fixed.

Change-Id: Ic6fd3628ba606142d3339da125e467b5fc6c16e3