tests: Add unit tests for cgroup_get_cgroup()

tests: Add unit tests for cgroup_get_cgroup()

This commit adds a unit test for cgroup_get_cgroup().  To
facilitate this, it creates a pseudo-cgroup sysfs in the
working directory and updates the test's cg_mount_table[]
to point at this temporary directory.

[----------] 2 tests from CgroupGetCgroupTest
[ RUN      ] CgroupGetCgroupTest.CgroupGetCgroup1
[       OK ] CgroupGetCgroupTest.CgroupGetCgroup1 (3 ms)
[ RUN      ] CgroupGetCgroupTest.CgroupGetCgroup_NoTasksFile
[       OK ] CgroupGetCgroupTest.CgroupGetCgroup_NoTasksFile (2 ms)
[----------] 2 tests from CgroupGetCgroupTest (6 ms total)

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>