gunit: Add unit tests for cgroup_process_v1_mnt()

gunit: Add unit tests for cgroup_process_v1_mnt()

This commit adds unit tests for cgroup_process_v1_mnt().  Note
that the compiler flag -Wno-write-strings was also added because
C++ throws a warning due to how the controllers[] array is
utilized.

[----------] 4 tests from CgroupProcessV1MntTest
[ RUN      ] CgroupProcessV1MntTest.AddV1Mount
[       OK ] CgroupProcessV1MntTest.AddV1Mount (0 ms)
[ RUN      ] CgroupProcessV1MntTest.AddV1Mount_Duplicate
[       OK ] CgroupProcessV1MntTest.AddV1Mount_Duplicate (0 ms)
[ RUN      ] CgroupProcessV1MntTest.AddV1NamedMount
[       OK ] CgroupProcessV1MntTest.AddV1NamedMount (0 ms)
[ RUN      ] CgroupProcessV1MntTest.AddV1NamedMount_Duplicate
[       OK ] CgroupProcessV1MntTest.AddV1NamedMount_Duplicate (0 ms)
[----------] 4 tests from CgroupProcessV1MntTest (0 ms total)

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>