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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[rust-log-analyzer]

The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

fmt: checked 6526 files
tidy check
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy: Skipping binary file check, read-only filesystem
tidy [style (tests)]: /checkout/tests/ui/proc-macro/auxiliary/nonfatal-parsing-body.rs:62: trailing whitespace
tidy [style (tests)]: FAIL
tidy: The following check failed: style (tests)
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 npm` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1549:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1279:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Build completed unsuccessfully in 0:00:49
  local time: Tue Nov  4 22:36:41 UTC 2025
  network time: Tue, 04 Nov 2025 22:36:41 GMT
##[error]Process completed with exit code 1.