If one is running avocado as a privileged user, the "usable_ro_dir()"
function may create a system wide directory for hosting tests.  That's
confusing because, the following workflow is possible:

 1) Regular user creates test, say "foo.py" in his own test dir
 2) Privileged user run "avocado run does_not_exist.py", which by
    means of "usable_ro_dir()", may create a system wide location
    for tests.
 3) Regular user runs "avocado run foo.py", and then test is not found
    because "avocado.core.data_dir.get_test_dir()" gives precedence to
    the system test dir created earlier.

This is also a problem when running the selftests from within the
tree, when working as a privileged user.  And finally, as a general
rule, we should avoid creating directories in system-wide locations so
silently, so I do believe this pattern could be expanded to the
results and data directories too.
Signed-off-by: NCleber Rosa <crosa@redhat.com>