Make static functions unit-testable

[stze]

static functions are currently not testable. we need some improvements to make this happen:

• Add a macro to disable static keyword in function prototype and declaration:

#ifdef BOX_UNIT_TESTS
#define STATIC
#else
#define STATIC static
#endif

• enable BOX_UNIT_TESTS compile option in sb-test cmake target
• move prototypes of static functions to a header file, e.g. for connection.c we would add a connection.h header file and add all prototypes:

STATIC void close_cb(uv_handle_t *handle);
STATIC int connection_handle_request(struct connection *con,
    msgpack_object *obj);
STATIC int connection_handle_response(struct connection *con,
    msgpack_object *obj);
STATIC void connection_request_event(connection_request_event_info *info);
STATIC void connection_close(struct connection *con);
STATIC int parse_cb(inputstream *istream, void *data, bool eof);

• include connection.h in test, finished

[mkind]

@stze, can we close the issue?