build: untangle `UNITTESTS` and `DEBUGBUILD` macros

build: untangle `UNITTESTS` and `DEBUGBUILD` macros

- fix `DEBUGBUILD` guards that should be `UNITTESTS`, in libcurl code
  used by unit tests.
- fix guards for libcurl functions used in unit tests only.
- sync `UNITTEST` attribute between declarations and definitions.
- drop `DEBUGBUILD` guard from test `unit2600`.
- fix guards for libcurl HSTS code used by both a unit test (`unit1660`)
  and `test0446`.
- update an existing AppVeyor CI job to test the issues fixed.

This fixes building tests with `CURLDEBUG` enabled but `DEBUGBUILD`
disabled. This can happen when building tests with CMake with
`ENABLE_DEBUG=ON` in Release config, or with `ENABLE_CURLDEBUG=ON`
and _without_ `ENABLE_DEBUG=ON`. Possibly also with autotools
when using `--enable-curldebug` without `--enable-debug`.

Test results:
- before:
  https://ci.appveyor.com/project/curlorg/curl/builds/49835609
  https://ci.appveyor.com/project/curlorg/curl/builds/49898529/job/k8qpbs8idby70smw
  https://github.com/curl/curl/actions/runs/9259078835/job/25470318167?pr=13798#step:13:821
- after: https://ci.appveyor.com/project/curlorg/curl/builds/49839255
  (the two failures are unrelated, subject to PR #13705)

Ref: #13592 (issue discovery)
Ref: #13689 (CI testing this PR with `DEBUGBUILD`/`CURLDEBUG` combinations)
Closes #13694