test1545: disable deprecation warnings

Fixes:
https://ci.appveyor.com/project/curlorg/curl/builds/48631551/job/bhx74e0i66yrp6pk#L1205

Same with details:
https://ci.appveyor.com/project/curlorg/curl/builds/48662893/job/ol8a78q9gmilb6wt#L1263
```
tests/libtest/lib1545.c:38:3: error: 'curl_formadd' is deprecated: since 7.56.0. Use curl_mime_init() [-Werror=deprecated-declarations]
   38 |   curl_formadd(&m_formpost, &lastptr, CURLFORM_COPYNAME, "file",
      |   ^~~~~~~~~~~~
[...]
```

Follow-up to 07a3cd8 curl#12421

Fixes curl#12445
Closes curl#12444

tests/libtest/lib1545.c

@@ -21,6 +21,9 @@
  * SPDX-License-Identifier: curl
  *
  ***************************************************************************/
+#ifndef CURL_DISABLE_DEPRECATION
+#define CURL_DISABLE_DEPRECATION  /* Using and testing the form api */
+#endif
 #include "test.h"
 
 int test(char *URL)