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

lib/doh.h

@@ -141,18 +141,19 @@ struct dohentry {
 };
 
 
-#ifdef DEBUGBUILD
-DOHcode doh_encode(const char *host,
-                   DNStype dnstype,
-                   unsigned char *dnsp, /* buffer */
-                   size_t len,  /* buffer size */
-                   size_t *olen); /* output length */
-DOHcode doh_decode(const unsigned char *doh,
-                   size_t dohlen,
-                   DNStype dnstype,
-                   struct dohentry *d);
-void de_init(struct dohentry *d);
-void de_cleanup(struct dohentry *d);
+#ifdef UNITTESTS
+UNITTEST DOHcode doh_encode(const char *host,
+                            DNStype dnstype,
+                            unsigned char *dnsp,  /* buffer */
+                            size_t len,  /* buffer size */
+                            size_t *olen);  /* output length */
+UNITTEST DOHcode doh_decode(const unsigned char *doh,
+                            size_t dohlen,
+                            DNStype dnstype,
+                            struct dohentry *d);
+
+UNITTEST void de_init(struct dohentry *d);
+UNITTEST void de_cleanup(struct dohentry *d);
 #endif
 
 extern struct curl_trc_feat Curl_doh_trc;