relate test-code conflicts between files

I was in the middle of setting up the CI test following the script

```
run_yamls=()
for i in $( git grep --name-only PythonCodeQuestion *.yml ) ; do
# FIXME: https://github.com/inducer/relate/issues/951
if ! grep data_files "$i" > /dev/null ; then
run_yamls+=("$i")
fi
done

relate test-code "${run_yamls[@]}"
```

This ends up reusing the module from previous yml. To be specific, `np.linalg.qr = not_allowed` is set inside
```
problem-bank/homework-problems/eigenvalue/code-qr-iteration-givens.yml
```
Then, when running the next file, `code-qr-iteration-shift.yml`, the call of `np.linalg.qr` triggers the assertion error.

Start with the shown `relate test-code "${run_yamls[@]}"` command and reproduce the failure using `problem-bank/homework-problems/eigenvalue/code-qr-iteration-givens.yml` followed by `code-qr-iteration-shift.yml`. Trace how `relate test-code` loads each YAML and determine why the module state from the first file is reused. Done means the two files can run in sequence without the `np.linalg.qr` assertion error.

Written by the indexing model from the issue text.