[delta-chat] Python unit tests for adapted Delta Chat core code

I have added a new feature to Delta Chat core, namely the [detection and removal of subject prefixes]. I prefer to have unit tests early during the development and I didn't find any existing ones in the repository. So I used [MinUnit] as a basis to quickly set up a C test framework and wrote my tests with that. In the meantime, I have asked around and have been pointed to the Python tests where I've been told that the C methods would automatically be available via `deltachat.capi.lib`. I ported my tests to Python, but they don't work because I can only access the existing C methods, not my added ones. I think it's because I followed the Readme which instructs to install the bindings via Pip so they can't possibly contain my code. But also when I analyse the symbols in the generated `python/src/deltachat/capi.*.so` files, my methods are missing. I should note that I added my new files to the Ninja build, the methods are contained in the compiled `build/src/libdeltachat.so` and I called `sudo ninja install` as mentioned in the Readme. My two questions therefore are: 1. How to make the compiled `capi*.so` files reflect the current C code in the repository? 2. How to use these bindings in the tests? [detection and removal of subject prefixes]: https://github.com/deltachat/deltachat-core/issues/126 [MinUnit]: http://www.jera.com/techinfo/jtns/jtn002.html