test: add a functional test suite

[amineau]

The project had no tests, which made any change to the protocol handling a
matter of trusting a manual session. This adds a suite that starts a real
server against a fixture root and drives it over a socket, plus end-to-end
cases through the client binary. Run it with make test.

Driving the protocol directly rather than through ./client is what lets the
suite send deliberately malformed input and assert on exact reply codes; the
client exits on the first unexpected reply, so it cannot express those cases.

Cases covering behaviour the server does not have yet are marked as expected
failures rather than left out. They run on every invocation and document the
intended behaviour, and fixing the code reports them as unexpected successes,
which fails the run and prompts removing the marker.

Three defects surfaced while writing this and are encoded as expected
failures:

• The working directory leaks between sessions. Workers chdir() the process
  and never reset, so a client can start in whatever directory the previous
  one left behind. Which directory depends on which worker accepts the
  connection, so the symptom is intermittent.
• ft_indexof cannot report "not found": it returns -1 only for a NULL
  haystack, and otherwise reads past the end of the array until it meets a
  matching byte. TYPE, MODE and STRU accept junk arguments as a result.
• TYPE, MODE and STRU dereference their argument without checking it, so a
  bare verb kills the worker handling the connection.

The harness pins the working directory when opening a connection so that the
first of these does not make execution order significant; a dedicated case
covers the defect itself.