In a (non-public) development branch the add subcommand was broken
although t0004-server exercises this subcommand and reported no
error. The bug went unnoticed because it would only bite when the
subcommand was given a directory to add, but the test specifies a
pathname which refers to a regular file.
To detect such breakage early, add a test which tries to add a
directory and another which exercises the rm command.