support/testing/tests/core/test_selinux: new tests for the packages SELinux functiona...

support/testing/tests/core/test_selinux: new tests for the packages SELinux functionalities

Add tests to ensure the packages SELinux functionalities (being able to
select an extra SELinux module in the refpolicy, and being able to
provide a custom SELinux module) are working as expected.

We use a BR2_EXTERNAL folder, provided in the tests, to use a custom
SELinux enabled package.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>