Test StringProducer implements required interface methods

Test exit from su'd shell returns to original user.

Test su --help output.

Test interactive su to phil creates new shell with phil identity.

Test su with invalid option shows error.

Test su - phil creates login shell with reset environment.

Test su - sets HOME to target user's home.

Test su - sets USER and LOGNAME.

Test su to a nonexistent user fails with error.

Test non-login su updates USER/LOGNAME/HOME.

Test su -c id phil shows phil's uid/gid.

Test su -c 'command' phil to run a command as phil.

Test su to root when already root (no password needed).

Test su --version output.

Test su -c 'command' to run a single command as root.

Test URL submission using v3 API format

Test subnegotiation command bytes.

A subshell between two commands keeps bash's output order.

Test basic subshell execution with parentheses - should output directly

Test subshell vs command substitution difference

Test parentheses with multiple commands - should execute all commands

Test parentheses in middle of command line is syntax error

Test command substitution does substitute output into command line

Test complex subshell with pipes syntax error - regression test

A subshell after a semicolon runs in order, like bash.

Test subshell with different command separators

`||` short-circuits: the first command succeeds, so the second is skipped (like bash).

Test subshell with multiple semicolons

A command with empty input inside $(...) must exit cleanly. The cmdstack must return to its original depth (no leaked shell), and the

Test successful TFTP download

Test successful FTP download with anonymous login

Test successful FTP download with username/password

Test that TELNET_OPTIONS contains expected values.

Test tftp with only hostname shows usage

Test tftp with invalid local directory

Test tftp command without arguments shows usage