nixos/sudo: fix test for 1.9.9

The test failed with

> Test "test5 user should not be able to run commands under root" failed with
> error: "invalid literal for int() with base 10: ''"

since 2492da88ea.

The reason for this is that `sudo(8)` writes the lecture to the
tty[1] and only as a fallback to stdout[2]. This means that the
`base64 --wrap 0` executed by `machine.execute()` doesn't affect the
text written to the terminal, however the lecture is part of the string
that's read from the VM via `shell.recv()`.

I confirmed the problem in an interactive test session[3]:

    >>> command = "sudo -u test5 sudo -n -u root true"
    >>> out_command = f"( set -euo pipefail; {command} ) | (base64 --wrap 0; echo)\n"
    >>> machine.shell.send(out_command.encode())
    84

    >>> machine # [   99.015512] sudo[877]:     root : TTY=hvc0 ; PWD=/tmp ; USER=test5 ; COMMAND=/run/wrappers/bin/sudo -n -u root true
    machine # [   99.019373] sudo[877]: pam_unix(sudo:session): session opened for user test5(uid=1005) by (uid=0)
    machine # [   99.038692] sudo[879]: pam_unix(sudo:auth): conversation failed
    machine # sudo: a password is required
    machine # [   99.041860] sudo[879]: pam_unix(sudo:auth): auth could not identify password for [test5]
    machine # [   99.046901] sudo[877]: pam_unix(sudo:session): session closed for user test5
    >>>
    >>> x=machine._next_newline_closed_block_from_shell()
    >>> print(x)
    <newline>
    We trust you have received the usual lecture from the local System
    Administrator. It usually boils down to these three things:
    <newline>
        #1) Respect the privacy of others.
        #2) Think before you type.
        #3) With great power comes great responsibility.
    <newline>
    <newline>
    <newline>
    >>>

Since the lecture isn't strictly necessary to confirm that
`security.sudo` works as expected, I decided to disable lecturing
inside the test, however we may want to fix the underlying problem in
the test-driver at some point.

[1] https://github.com/sudo-project/sudo/blob/SUDO_1_9_9/plugins/sudoers/check.c#L275-L283
[2] https://github.com/sudo-project/sudo/blob/SUDO_1_9_9/src/conversation.c#L95-L120
[3] I replaced each empty line with `<newline>` to make sure these
    aren't swallowed by git.

nixos/tests/sudo.nix

@@ -28,6 +28,10 @@ in
           enable = true;
           wheelNeedsPassword = false;
 
+          extraConfig = ''
+            Defaults lecture="never"
+          '';
+
           extraRules = [
             # SUDOERS SYNTAX CHECK (Test whether the module produces a valid output;
             # errors being detected by the visudo checks.