Update offload test and verify that tgt_(un)register_lib have the right type
Apparently, we weren't running offload tests when Enzyme wasn't built. Time to fix that.
Also adds a test mode which generates the host IR, but does not expect device IR/artifacts. This way, we don't have to handle artifacts and paths in our tests.
Also removes some outdated documentation.
cc `@Kevinsala,` `@Sa4dUs`
closes: https://github.com/rust-lang/rust/issues/150415
~~blocked on `needs-offload` infrastructure landing in https://github.com/rust-lang/rust/pull/150427~~
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