Add unittest

[   23s] + ./num_cpus
[   23s]
[   23s] running 15 tests
[   23s] test linux::tests::v1::test_cgroup_cpu_quota ... ok
[   23s] test linux::tests::v1::test_cgroup_cpu_quota_ceil ... ok
[   23s] test linux::tests::v1::test_cgroup_cpu_quota_divide_by_zero ... ok
[   23s] test linux::tests::v1::test_cgroup_mount ... ok
[   23s] test linux::tests::v1::test_load_mountinfo ... ok
[   23s] test linux::tests::v1::test_load_subsys ... ok
[   23s] test linux::tests::v2::test_cgroup_cpu_quota ... ok
[   23s] test linux::tests::v2::test_cgroup_cpu_quota_ceil ... ok
[   23s] test linux::tests::v2::test_cgroup_cpu_quota_divide_by_zero ... ok
[   23s] test linux::tests::v2::test_cgroup_mount ... ok
[   23s] test linux::tests::v2::test_load_mountinfo ... ok
[   23s] test linux::tests::v2::test_load_subsys ... ok
[   23s] test linux::tests::v2::test_load_subsys_multi ... ok
[   23s] test tests::test_get ... ok
[   23s] test tests::test_get_physical ... ok
[   23s]
[   23s] test result: ok. 15 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.05s

Bump to rust-num_cpus 1.15.0

Import num_cpus 1.15.0