cgroup_test: Make user can run part of test cases

In the future there will be several test cases in cgroup_test
module, and they have no relationship. For now, these cases just
run integrally, user cannot run desired case. So I do some modify
to adapt this situation.

1.test.cfg can desire which test case you want to run.
2.Defaultly test.cfg contains all the testcases, you
  can modify its detail to run different case.
3.test-example.cfg contains all the testcases, you
  can get every testcase name.(No need to modify it)

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>