Given that the setUp() phase is not useful for test_disk_space, and the
custom cleanup done in the test is better had at the tearDown() phase.

Additionally, the following changes where made to make it more reliable:

 * Avoid the busy loop giving modprobe some time between attempts
 * Use "modprobe -r" instead of rmmod, which handlers module dependencies
 * Do an explicit check for status
 * If "modprobe -r" doesn't succeed, it's not a test failure, but a test
   a test error (given that the get_diskspace() function already ran
   and succeeded).

Ideally, the module removal would be done by the linux_modules.unload_module()
but there are some issues with that function that needs addressing first
(such as returning a meaningful result).
Signed-off-by: NCleber Rosa <crosa@redhat.com>