./x.py test does not explain why a #[should_panic] test failed

[Xiretza]

When an stdlib #[should_panic] test does not actually panic, ./x.py test fails but gives no explanation as to why, even with -vv:

$ ./x.py test library/core/ -vv
[snip]
     Running `$HOME/dev/rust/rustc/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/deps/coretests-207451fa6fb9ecc4 --quiet -Z unstable-options --format json`

running 1596 tests
........................................................................................   88/1596
........................................................................................  176/1596
........................................................................................  264/1596
........................................................................................  352/1596
........................................................................................  440/1596
........................................................................................  528/1596
........................................................................................  616/1596
.....................................................ii.................................  704/1596
........................................................................................  792/1596
........................................................................................  880/1596
........................................................................................  968/1596
........................................................................................ 1056/1596
........................................................................................ 1144/1596
........................................................................................ 1232/1596
........................................................................................ 1320/1596
................................................................FFFF.................... 1408/1596
........................................................................................ 1496/1596
........................................................................................ 1584/1596
............

failures:


failures:
    slice::slice_rsplit_array_ref_out_of_bounds
    slice::slice_rsplit_array_mut_out_of_bounds
    slice::slice_split_array_mut_out_of_bounds
    slice::slice_split_array_ref_out_of_bounds

test result: FAILED. 1590 passed; 4 failed; 2 ignored; 0 measured; 0 filtered out; finished in 1.28s

error: test failed, to rerun pass `-p core --test coretests`

The instructions at the end also don't work, ./x.py test doesn't have -p or --test parameters.

[ehuss]

Looks like this was a small oversight from #108659. I have posted a fix for this.