Add two new tests for para_server.

Add two new tests for para_server.

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.