The introduction of the `extended(astgen_error())` instruction allows a
`test` declaration to be unresolved, i.e. the declaration doesn't even
contain a `func`. I could modify AstGen to not do this, but it makes
more sense to just handle this case when collecting test functions.

Note that tests under incremental compilation are currently broken if
you ever remove all references to a test; this is tracked as a subtask
of #21165.