It is unclear how to reproduce "Testing stage2 error-index (x86_64-unknown-linux-gnu)"

In this CI run, I got the following error:

Testing stage2 error-index (x86_64-unknown-linux-gnu)
STDOUT:

running 1106 tests
[...]

failures:
    /checkout/obj/build/x86_64-unknown-linux-gnu/test/error-index.md - Rust_Compiler_Error_Index::E0094 (line 1995)
    /checkout/obj/build/x86_64-unknown-linux-gnu/test/error-index.md - Rust_Compiler_Error_Index::E0211::_::Note__this_error_code_is_no_longer_emitted_by_the_compiler_ (line 3999)

test result: FAILED. 1043 passed; 2 failed; 61 ignored; 0 measured; 0 filtered out; finished in 8.56s


Command CFG_RELEASE_CHANNEL="nightly" RUSTC_BOOTSTRAP="1" RUSTC_STAGE="2" RUSTC_SYSROOT="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2" RUSTDOC_LIBDIR="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" RUSTDOC_REAL="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustdoc" RUST_TEST_THREADS="16" "/checkout/obj/build/bootstrap/debug/rustdoc" "-Wrustdoc::invalid_codeblock_attributes" "-Dwarnings" "-Znormalize-docs" "-Z" "unstable-options" "--test" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/error-index.md" "--test-args" "" (failure_mode=DelayFail) has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:41:18

I have no idea how to reproduce this locally. "Testing stage2 error-index" sounds like it should be ./x test error-index, but that does not work. So right now I am making guesses and pushing that to the PR, leading to a 40min round-trip time for an edit-compile cycle...

In general, when an x.py job fails, it'd be really nice if it could, at the end of the error log, give some idea of how this can be reproduced -- for instance, it can also be non-trivial to find out which target the failing test ran on.

Cc @rust-lang/bootstrap