Most CI jobs run an OS that does not support SELinux, therefore tests that
need it should be skipped by default.
* [Feature:SELinux] marks tests that need SELinux (for any feature)
* [Feature:SELinuxMountReadWriteOncePod] marks tests that need
SELinuxMountReadWriteOncePod alpha gate enabled.
Currently, all SELinux tests have both, but it will change in the future.