Tests do not use the preCICE API

[IshaanDesai]

Currently the tests do not use the C bindings API of preCICE, but rather a dummy C API. This makes the testing infrastructure easy to maintain, but also quite weak, as highlighted in #53. Only the solver dummy tests against a real API, and unfortunately it does not cover all the API functions. There is a need for tests which will use the C bindings API and test individual functions.