test1545: disable deprecation warnings

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 07a3cd83e0456ca17dfd8c3104af7cf45b7a1ff5 #12421

Fixes #12445
Closes #12444