modem-helpers: rework AT string quote operation and add unit tests

Rework the AT string quote operation to build the output with the
GString helper, instead of manually calculating how many bytes we'll
need in the output. It just makes it clearer.

src/plugins/option/mm-broadband-bearer-hso.c

@@ -557,8 +557,8 @@ authenticate (GTask *task)
             return;
         }
 
-        quoted_user     = mm_port_serial_at_quote_string (user);
-        quoted_password = mm_port_serial_at_quote_string (password);
+        quoted_user     = mm_at_quote_string (user);
+        quoted_password = mm_at_quote_string (password);
         command = g_strdup_printf ("%s=%d,%u,%s,%s",
                                    auth_commands[ctx->auth_idx],
                                    ctx->cid,