Add a regression test for trailing attributes in doctests

[JonathanBrouwer]

Adds a regression test for #162131 and #162129

It seems likely this breakage is going to be accepted, so let's add a test in nightly so we can at least track this behavior in the feature

[rustbot]

r? @lolbinarycat

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[mejrs]

#162352 #162352 have some more variants of this, can you add these as well?

[rust-log-analyzer]

The job test-aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

To only update this specific test, also pass `--test-args expect-item-after-attribute.rs`

error: 1 errors occurred comparing output.
status: exit status: 101
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustdoc" "/checkout/tests/rustdoc-ui/expect-item-after-attribute.rs" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-o" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/rustdoc-ui/expect-item-after-attribute" "-Znext-solver=coherence" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Cdebuginfo=0" "--test" "--test-args=--test-threads=1"
--- stdout -------------------------------

running 5 tests
test /checkout/tests/rustdoc-ui/expect-item-after-attribute.rs - (line 12) ... FAILED
test /checkout/tests/rustdoc-ui/expect-item-after-attribute.rs - (line 16) ... FAILED
---
---- /checkout/tests/rustdoc-ui/expect-item-after-attribute.rs - (line 19) stdout ----
error: expected item, found keyword `let`
##[error]  --> /checkout/tests/rustdoc-ui/expect-item-after-attribute.rs:20:1
   |
LL | let x = 0; //! assert!(true);
   | ^^^
   | |
   | `let` cannot be used for global variables
   | help: consider using `static` or `const` instead of `let`
   |

[JonathanBrouwer]

I will fight CI & include the testcases mejrs linked
@rustbot author