ServletTestRunner is leaking memmory

ServletTestRunner saves ThreadLocal reference to ServletContex in its init() method,
and clears it in destroy().
Problem is that thread that calls init and thread that call destroy() are not guaranteed to be the same thread.

In wildfly/undertow undeploy/server shotdown is executed in different thread than he one that was used to call servlet init first.

reproducer and some evaluation on the bug can be found at https://github.com/wildfly/wildfly/issues/8263