build: fix some -Wsign-conversion/-Warith-conversion warnings

- enable `-Wsign-conversion` warnings, but also setting them to not
  raise errors.
- fix `-Warith-conversion` warnings seen in CI.
  These are triggered by `-Wsign-converion` and causing errors unless
  explicitly silenced. It makes more sense to fix them, there just a few
  of them.
- fix some `-Wsign-conversion` warnings.
- hide `-Wsign-conversion` warnings with a `#pragma`.
- add macro `CURL_WARN_SIGN_CONVERSION` to unhide them on a per-build
  basis.
- update a CI job to unhide them with the above macro:
  https://github.com/curl/curl/actions/workflows/linux.yml -> OpenSSL -O3

Closes #12492

tests/libtest/testutil.c

@@ -37,8 +37,8 @@ struct timeval tutil_tvnow(void)
   */
   struct timeval now;
   DWORD milliseconds = GetTickCount();
-  now.tv_sec = milliseconds / 1000;
-  now.tv_usec = (milliseconds % 1000) * 1000;
+  now.tv_sec = (long)(milliseconds / 1000);
+  now.tv_usec = (long)((milliseconds % 1000) * 1000);
   return now;
 }