gunit: Add unit tests for cgroupv2_subtree_control()
This commit adds unit tests for cgroupv2_subtree_control().
[----------] 2 tests from SubtreeControlTest
[ RUN ] SubtreeControlTest.AddController
[ OK ] SubtreeControlTest.AddController (0 ms)
[ RUN ] SubtreeControlTest.RemoveController
[ OK ] SubtreeControlTest.RemoveController (0 ms)
[----------] 2 tests from SubtreeControlTest (0 ms total)
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>