tests: Add unit tests for cg_build_path()

tests: Add unit tests for cg_build_path()

This commit adds unit tests for the internal function
cg_build_path().  While code coverage is not (yet) enabled,
I believe these tests provide full code coverage for the
aforementioned function and its sister function
cg_build_path_locked().

The following tests are in this commit:

BuildPathV1_ControllerMismatch() - Calls cg_build_path() with a
    controller that isn't in cg_mount_table[].  Expects a return
    value of NULL

BuildPathV1_ControllerMatch() - Calls cg_build_path() with a valid
    controller in cg_mount_table[]

BuildPathV1_ControllerMatchWithName() - Calls cg_build_path() with
    a valid controller and a cgroup name

BuildPathV1_ControllerMatchWithNs() - Calls cg_build_path() with a
    valid controller that has a namespace

BuildPathV1_ControllerMatchWithNameAndNs() - Calls cg_build_path()
    with a valid controller, a cgroup name, and the controller has
    a namespace

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>