test: migrate to `vitest` (#14337)

Continuation of https://github.com/payloadcms/payload/pull/10200

* For VSCode - you'll need to install the official plugin
https://marketplace.visualstudio.com/items?itemName=vitest.explorer
* Every test suite now explicitly imports the test runner functions like
`describe`, `expect`, instead of using it from `global`. While `vitest`
supports globals with https://vitest.dev/config/#globals, the main
benefit of not doing this is that we don't conflict with test runners
that have the same functions, e.g `playwright` and `tstyche`.
* Now we have only 1 test config file that uses
[projects](https://vitest.dev/guide/projects) for unit and integration
tests

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Co-authored-by: Alessio Gravili <github@gravili.net>