6ln RA PIO with L flag set is not ignored

[xnumad]

Description

(This was noticed because of the tests failing in #20371 )

A router MUST NOT set the L (on-link) flag in the PIOs, since that
might trigger hosts to send multicast NSs.

• https://datatracker.ietf.org/doc/html/rfc6775#section-6.1

Should the host erroneously receive a PIO with the L (on-link) flag
set, then that PIO MUST be ignored.

• https://datatracker.ietf.org/doc/html/rfc6775#section-5.4

The latter quote is not implemented in either tests or tested code.

Code implementation (tested code) adjustment would be a new PIO validity check failure condition: || (gnrc_netif_is_6ln(netif) && (pio->flags & NDP_OPT_PI_FLAGS_L))
Test adjustment is slightly more changes.

Steps to reproduce the issue

For example, observe failing tests in #20371

Expected results

These tests that currently fail in #20371 should not do so.

Actual results

They do fail.

Versions

Murdock on #20371