In PHP 8.2 and later, setting a value to an undeclared class property is
deprecated and emits a deprecation notice.
So we need to add missing class properties that still need to be declared.
While working on this issue, even though it's not related to the deprecation issue,
I added 'use core_external\external_api' to avoid the 'Undefined type external_api' warning at
mod_lti_testcase::generate_tool_proxy().