`diagnostics_dont_block_typing` test is flaky

    failures:
        diagnostics_dont_block_typing

    test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 34 filtered out; finished in 42.17s

  stderr ───

    thread 'diagnostics_dont_block_typing' panicked at crates\rust-analyzer\tests\slow-tests\main.rs:938:5:
    typing enter took 3.2540789s
    stack backtrace:
       0: std::panicking::begin_panic_handler
                 at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library\std\src\panicking.rs:697
       1: core::panicking::panic_fmt
                 at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library\core\src\panicking.rs:75
       2: slow_tests::diagnostics_dont_block_typing
                 at .\tests\slow-tests\main.rs:938
       3: slow_tests::diagnostics_dont_block_typing::closure$0
                 at .\tests\slow-tests\main.rs:882
       4: core::ops::function::FnOnce::call_once<slow_tests::diagnostics_dont_block_typing::closure_env$0,tuple$<> >
                 at C:\Users\runneradmin\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\ops\function.rs:250
       5: core::ops::function::FnOnce::call_once
                 at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library\core\src\ops\function.rs:250
    note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[> 60.000s] rust-analyzer::slow-tests completes_items_from_standard_library
[> 60.000s] rust-analyzer::slow-tests resolve_proc_macro
────────────
     Summary [ 142.236s] 7085 tests run: 7084 passed (2 slow), 1 failed, 7 skipped
        FAIL [  42.204s] rust-analyzer::slow-tests diagnostics_dont_block_typing

We got this test failing more often these days

Start with crates/rust-analyzer/tests/slow-tests/main.rs, especially diagnostics_dont_block_typing around lines 882 and 938, and reproduce the reported failure where typing enter takes 3.254 seconds. Compare the test's timing behavior with the other slow tests. Done means diagnostics_dont_block_typing runs reliably without false failures while still checking that diagnostics do not block typing.