Remove uname Help override, add test

[julesmcrt]

Builtins can have a Help field.
We want to ensure consistency between help command and command --help.
Only the commands with NoFlags should be able to override the Help field, adding a test to check that.

See https://man7.org/linux/man-pages/man1/uname.1.html

Before:

❯ ./rshell --allow-all-commands -c 'help uname'
uname: uname [-asnrvm]
    Print system information.

    With no flags, print the kernel name (same as -s).
    Reads from /proc/sys/kernel/ (configurable via --proc-path).

❯ ./rshell --allow-all-commands -c 'uname --help'
Usage: uname [OPTION]...
Print system information. With no OPTION, same as -s.

  -s, --kernel-name     print the kernel name
  -n, --nodename        print the network node hostname
  -r, --kernel-release  print the kernel release
  -v, --kernel-version  print the kernel version
  -m, --machine         print the machine hardware name
  -a, --all             print all information
      --help            display this help and exit

After:

❯ ./rshell --allow-all-commands -c 'uname --help'
Usage: uname [OPTION]...
Print system information. With no OPTION, same as -s.

  -s, --kernel-name     print the kernel name
  -n, --nodename        print the network node hostname
  -r, --kernel-release  print the kernel release
  -v, --kernel-version  print the kernel version
  -m, --machine         print the machine hardware name
  -a, --all             print all information
      --help            display this help and exit

❯ ./rshell --allow-all-commands -c 'help uname'
Usage: uname [OPTION]...
Print system information. With no OPTION, same as -s.

  -s, --kernel-name     print the kernel name
  -n, --nodename        print the network node hostname
  -r, --kernel-release  print the kernel release
  -v, --kernel-version  print the kernel version
  -m, --machine         print the machine hardware name
  -a, --all             print all information
      --help            display this help and exit