When constructing the protocol list in the class metadata generation
(`GenerateClass`), only the protocols from the base class are added but
not protocols declared in class extensions.
This is fixed by using `all_referenced_protocol_{begin, end}` instead of
`protocol_{begin, end}`, matching the behaviour on Apple platforms.
A unit test is included to check if all protocol metadata was emitted
and that no duplication occurs in the protocol list.
Fixes https://github.com/gnustep/libobjc2/issues/339
CC: @davidchisnall
