tests: fix callback signatures to please UndefinedBehaviorSanitizer

Make test applications use the correct prototypes for callbacks.

Closes #15289

libtest: 2308 verifies CURLE_WRITE_ERROR after write callback error

Verifies that the issue in #13669 actually is fixed. This return code is
what the CURLOPT_WRITEFUNCTION manpage docume

libtest: 2308 verifies CURLE_WRITE_ERROR after write callback error

Verifies that the issue in #13669 actually is fixed. This return code is
what the CURLOPT_WRITEFUNCTION manpage documents should be returned.

This code is mostly from the
Source-written-by: Trumeet on github
Closes #13671

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