add test for assoc type norm wf check

[lcnr]

This is soundness critical 😁 sure is helpful to have tests for that. cc @rust-lang/types

r? @BoxyUwU

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[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain enhanced) (plain)

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  PR_CI_JOB: 1
  SKIP_SUBMODULES: src/gcc
  IMAGE: tidy
##[endgroup]
error: PRs changing the `nightly` channel should be sent to the `master` branch!
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"
echo "disk usage:"
df -h
shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}

[BoxyUwU]

"This is necessary for soundness until we properly handle implied bounds on binders" or sth?

[lcnr]

hmm, I've already put this in the test, u want me to also add that comment here?

[BoxyUwU]

speaking for myself if i read these comments ill probably go "hmm that feels kinda iffy" and get confused and spend a few minutes wasting time before figuring out whats going on (e.g. from reading the test). i don't care too much though so feel free to r=me if you don't think its worth writing in the comments too