Configure CTest

[mrossinek]

This updates the C testing configuration to become a CMake/CTest build.

Key features are as follows:

• files matching test/c/test_*.c are autodetected as tests
• every such fail must contain one function which has the same name as the filename without extension
• make c-test will build a test runner collecting all of the tests above in test/c/build and execute those tests

Notes:

• the required match between the filename and test function is a caveat of this method 1. Alternatively, we must specify each test individually inside test/c/CMakeLists.txt
• I still need to figure out how to get find_library to work (see comment in test/c/CMakeLists.txt)
• test what happens when individual subtests fail and what that looks like from the CLI
• currently, test/c/test_main2.c is a copy of test/c/test_main.c to prototype the glob. We should make this more sensible as we develop the C API. We could do a recursive glob if we want to write multiple test files e.g. inside test/c/sparse_observable/test_*.c. Happy to discuss this.