tests: refactor remoteFileHandler truncateFile tests

The old test was too big, causing one test to fail if the other failed.
Now there are 2 separated tests, sharing setup, teardown and check
methods.

Additionally, creation of the test file is more correct, and the code is
hopefully more readable.

Change-Id: I0c23482e5d25651dc6090c7acfaa0ae221a562fe
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20084
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>