tests/gunit: Extend the fuzzer to test cgroup_set_value_string()

tests/gunit: Extend the fuzzer to test cgroup_set_value_string()

Add fuzzing to the cgroup_set_value_string() API, by passing combination
of valid/NULL as arguments to the API.

[----------] 3 tests from APIArgsTest
[ RUN      ] APIArgsTest.API_cgroup_set_permissions
[       OK ] APIArgsTest.API_cgroup_set_permissions (0 ms)
[ RUN      ] APIArgsTest.API_cgroup_new_cgroup
[       OK ] APIArgsTest.API_cgroup_new_cgroup (0 ms)
[ RUN      ] APIArgsTest.API_cgroup_set_value_string
[       OK ] APIArgsTest.API_cgroup_set_value_string (19 ms)
[----------] 3 tests from APIArgsTest (19 ms total)

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>