at_c: use Unity as our unit testing framework

[JeremyTubongbanua]

Right now in our unit tests, we simply do if-statement checks to check if a value is as expected.

It was discussed in #344 that we should make our own atassert library and even use it in our arguments validation in every function, but I think it is okay to just use another assertion library such as Unity and even make use of its features like mocks. As for arguments validation, if-statements are fine, since there aren't many of them.

Work involved:

• Create find_unity.cmake in ./cmake/
• Add Unity as part of our CMakeLists.txt
• Use assertion library in atchops unit tests
• Rename atchops tests accordingly to previous refactors and conventions
• Use assertion library in atclient unit tests
• Rename atclient tests accordingly to previous refactors and conventions