test: make sure we refer to local headers

The previous time I checked whether everything was compiling
smoothly, I was using installed headers.

Lesson learned. Make sure we refer the local headers.
Make sure ight_wrappers.h declares struct evbuffer

As a consequence, we avoid a compiler warning caused by `struct
evbuffer *` being referenced but not declared.

Guess this is the fault of relying on installed headers: I noted
that the first time I recompiled with the local headers.