Run a Codeshift to move away from the custom test runner

[yeoffrey]

Related: #1244

This PR does a few things to the testing suite:

• It removes the old test runner and instead turns each test, which was previously a configuration for a test, into its own vitest test.
• it adds a custom matcher to compare StructErrors, since there is extra logic on comparing StructErrors in the old test runner and its worth not redefining that check everywhere.

In a follow up I'd like to move all the tests into one file per struct, but that can be done by hand later.

[yeoffrey]

Here is the script for renaming the files to end with .test.ts

[yeoffrey]

Here is my script for running the Codeshift (warning its actually terrible but whatever)

[yeoffrey]

Adding a vitest configuration file for adding the custom matcher

[yeoffrey]

File for adding custom matchers, which includes .toMatchStructError

The old test runner would check for a failure, and check that the StructError gave the right failures but also that the StructError's properties (only the FILTERED_PROPS ones) matched the first Failure returned by the failures generator on the StructError