Our CMakeLists.txt is very unwieldy in its current size, spanning more
than 700 lines of code. Furthermore, it has several issues regarding
scoping, where for example some defines, includes, etc. from our test
suite are also applied to our normal library code.

To fix this, we can separate out build instructions for our tests and
move them into their own CMakeLists.txt in the "tests" directory. This
reduced complexity of the root CMakeLists.txt file and fixes the issues
regarding leaking build context from tests into the library.