fs-tests: integck: improve re-mounting test coverage

fs-tests: integck: improve re-mounting test coverage

The integck tests re-mounts the file-system from time to time
and checks the integrity afterwords. And it re-mounts always
the same-way: unmount and then mount R/W back. However, it is
better to do it differently some times, e.g.:

* re-mount R/O then re-mount R/W
* unmount then mount R/W
* both of the above
* unmount, mount R/O, then re-mount R/W
* etc.

This will give better test coverage. This patch does exactly
that by improving the 'tests_remount()' function.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>