ftp tests: provide LIST responses in the test file itself

Previously LIST always returned a fixed hardcoded list that the ftp
server code knew about, mostly since the server didn't get any test case
number in the LIST scenario. Starting now, doing a CWD to a directory
named test-[number] will make the test server remember that number and
consider it a test case so that a subsequent LIST command will send the
<data> section of that test case back.

It allows LIST tests to be made more similar to how all other tests
work.

Test 100 was updated to provide its own directory listing.