pipeline.md: mention propertywise-like tests

[markusicu]

TODO for @eggrobin from 2024-06-06 PAG meeting:

[𒉭] FYI: We can now verify statements like « this new character should be like that other one » on the draft data; compare this comment in an old PAG review of an SÆǶ proposal and this test (note that Identifier_Status Identifier_Type Idn_Status is effectively spam here, since we are not deriving those in the draft PRs).

1. Are we going to keep these new assertions after the new characters are encoded? Yes, for now, until they become annoying.
2. Egg to mention that in pipeline.md.