dgram tests/improvements

Also requires https://github.com/tessel/firmware/pull/104. Should be ready now.
Hrm. Currently we do have `int tm_tcp_read` (blithely cast to `size_t` in `l_tm_tcp_read` which'll make extra fun every now and then ;-) but also the comparable `ssize_t tm_ssl_write`/`…read` functions.

Today I like the idea of using any non-zero `int` to signal error status instead of the split state. I'll leave the ssl ones but will try clean up error handling a bit on both udp/tcp.