tests: make the unit test result type `CURLcode`

Before this patch, the result code was a mixture of `int` and
`CURLcode`.

Also adjust casts and fix a couple of minor issues fou

tests: make the unit test result type `CURLcode`

Before this patch, the result code was a mixture of `int` and
`CURLcode`.

Also adjust casts and fix a couple of minor issues found along the way.

Cherry-picked from #13489
Closes #13600

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lib1598: fix `CURLOPT_POSTFIELDSIZE` usage

It requires a `long` argument.

Closes https://github.com/curl/curl/pull/13085

tests: add test1598 for POST with trailers

- test POST fields with trailers and chunked encoding

Ref: #12938
Closes #13009