add tests for incremental-cache (#60331)
### What?
In the previous PR (#60249), it was found that the case when the
response size is larger than 2MB is not sufficiently covered by tests
[[comment](https://github.com/vercel/next.js/pull/60249#pullrequestreview-1807140518)]
### How?
I added a few more checks.
Just checking the number of API calls is not enough - I believe it is
important to additionally verify what exactly the page received.
If I'm honest, I couldn't find the exact reason why data is loaded after
application start when using `customCacheHandler`, but without it -
during build. It seems like something in the butcher. Therefore, in this
part, I simply configured it for the current version.
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Co-authored-by: JJ Kasper <jj@jjsweb.site>
