Rollup merge of #148505 - cyrgani:pm-tests, r=madsmtm

add larger test for `proc_macro` `FromStr` implementations

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 (rust-lang/rust#130856) which will probably need to reimplement this logic as well as for removing panics from these functions (rust-lang/rust#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 rust-lang/rust#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.

Author: Zalathar