tests/gunit: Add a simple fuzzer

tests/gunit: Add a simple fuzzer

Add a simple fuzzing test case that would exercise API's unlikely code
paths by passing NULL/negative values as arguments.

Output:
--------
[...]

[----------] 1 test from APIArgsTest
[ RUN      ] APIArgsTest.API_cgroup_set_permissions
[       OK ] APIArgsTest.API_cgroup_set_permissions (0 ms)
[----------] 1 test from APIArgsTest (0 ms total)

to begin with, it exercises cgroup_set_permissions() by passing a NULL
pointer in place of struct cgroup. This test case will be extended to
cover almost all APIs.

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