[BUGFIX] Fix test setup to get ephemeral context

[tyler-hoffman]

Fixes the failure seen here: https://github.com/great-expectations/great_expectations/actions/runs/7988746346/job/21814183950#step:5:233. I suspect there are other underlying issues with sharing context between tests, but following the error from CI makes it look like in this case, just forcing an ephemeral context does the trick.

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