add larger test for proc_macro FromStr implementations

[cyrgani]

Currently, there are only few tests that check the output of TokenStream::from_str and Literal::from_str (which is somewhat understandable as the rustc implementation just delegates these calls to the parser). In preparation for both the standalone backend (#130856) which will probably need to reimplement this logic as well as for removing panics from these functions (#58736), this PR adds a test which shows the various messy ways of how these functions report errors and the return values for successful parses.
Followup PRs such as #147859 will change more and more of these "diagnostic + error"s into LexErrors.

The test structure with the extra module is used to allow reusing it later easily for the standalone backend.

[rustbot]

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