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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unit2604: use alloc instead of overlong string const

Closes #13563

curl_path: make Curl_get_pathname use dynbuf

... instead of malloc and memcpy

- unit test 2604 verifies Curl_get_pathname()

Closes #13550