tests: metadata: fix failures on systems with selinux enabled

tests: metadata: fix failures on systems with selinux enabled

We get a number of failures if selinux is enabled:

! tmetadata.py:298  ['user.foo', 'security.selinux'] == ['user.foo']     FAILED
! tmetadata.py:302  ['user.foo', 'security.selinux'] == ['user.foo']     FAILED
! tmetadata.py:306  ['user.foo', 'security.selinux'] == ['user.foo']     FAILED

Filter out security.selinux to address that.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>