[lld-macho] Reorganize + extend ICF test

I found icf.s a bit hard to work with as it was not possible to
extend any of the functions `_a` ... `_k` to test new relocation /
referent types without modifying every single one of them. Additionally,
their one-letter names were not descriptive (though the comments
helped).

I've renamed all the functions to reflect the feature they are testing,
and shrunk them so that they contain just enough to test that one
feature.

I've also added tests for non-zero addends (via the
`_abs1a_ref_with_addend` and `_defined_ref_with_addend_1` functions).

Reviewed By: #lld-macho, gkm

Differential Revision: https://reviews.llvm.org/D106211