Add tests: round-trip with object, and compare to llvm-ar

[dpaoliello]

Adds tests based on the round-trip tests from object.

Verifies:

• Archives can be read with object and original object exactly matches what object read.
• Archive exactly matches what llvm-ar generates.

There were two interesting issues:

• 32-bit Darwin objects have an extra 4 bytes at the end (all 0x0F) but only when read with object.
• Found a bug in the Darwin implementation where we weren't resetting the unique timestamp to 0 if there was only 1 object with the same name (see LLVM: https://github.com/llvm/llvm-project/blob/5748ad84e5e8e5621f221199cc290666f00e2a30/llvm/lib/Object/ArchiveWriter.cpp#L815)

Fixes #3

[dpaoliello]

@bjorn3 I've also implemented tests that verify the output versus llvm-ar

[bjorn3]

32-bit Darwin objects have an extra 4 bytes at the end (all 0x0F) but only when read with object.

Looks like those extra bytes are real and intententional:

https://github.com/llvm/llvm-project/blob/3d3ef9d073e1e27ea57480b371b7f5a9f5642ed2/llvm/lib/Object/ArchiveWriter.cpp#L560-L565

[bjorn3]

LGTM with the extra newline. Thanks!