Make test client's fileUploadMutation work for more input variable shapes

[toBeOfUse]

Description

In the testing package, Vendure provides a SimpleGraphQLClient that has a function called fileUploadMutation. This function works for the built-in GraphQL mutations that accept file uploads, but if you add a custom mutation that accepts file inputs, as the docs show you how to do, you will run into problems, since it assumes that it can find the Upload inputs in the GraphQL variables just by looking at the first property of each nested object. So, for example, the existing code that creates the FormData body for the upload will go into an infinite loop if there are any string fields before the Upload fields in your GraphQL mutation input. I detailed the problem that I ran into with this in #1671.

So, this PR makes the fileUploadMutation method work with mutation inputs with arbitrary shapes.

Also, the two tests for this functionality that exist seem to be written for Jest, and there is no test command inside the testing package.json to run them. I converted the test file to Vitest and added the test command so that they could be run.

Breaking changes

None: the fileUploadMutation should work the same as it always had for the mutations it previously worked with, as shown by the two existing tests passing.

Checklist

📌 Always:

• I have set a clear title
• My PR is small and contains a single feature
• I have checked my own PR

👍 Most of the time:

• I have added or updated test cases
• I have updated the README if needed

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Name	Status	Preview	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	Nov 6, 2024 3:12am

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[michaelbromley]

Wow, I think this is my favourite PR of the week. Tricky investigation, complex code to fix, and even getting previously-lost tests back up and running again! Bravo and thanks so much for your continued contributions 🙌