GHA/macos: enable HTTPS tests with stunnel

- Install stunnel.

- Regenerate certificates (as SecureTransport requires a validity period
  less than 398 days).

- Restart server if it is unresponsive.

- Do not hardcode the SHA-256 base64 public pinned key.

- Ignore test 313 as SecureTransport does not support crl file.

- Ignore tests 1631 and 1632 as SecureTransport is not yet able to shut
  down FTP over HTTPS gracefully.

- Add a CMake target for generating certificates.

Closes #14486

tests/servers.pm

@@ -2432,8 +2432,9 @@ sub startservers {
             }
         }
         elsif($what eq "http") {
-            if($torture && $run{'http'} &&
+            if($run{'http'} &&
                !responsive_http_server("http", $verbose, 0, protoport('http'))) {
+                logmsg "* restarting unresponsive HTTP server\n";
                 if(stopserver('http')) {
                     return ("failed stopping unresponsive HTTP server", 3);
                 }