Document use of Jetty's WebSocketServerFactory

Issue: SPR-11023
Expose failing test in GenericMessagingTemplateTests

Assertions made in callbacks invoked on separate thread were not
reaching the test framework. This fix ensures failures are detected
and cause tests to fail.
Fix typo
Handle NoClassDefFoundError consistently for TELs

Prior to this commit, a NoClassDefFoundError caught in
TestContextManager's retrieveTestExecutionListeners() method would be
handled differently for implicit default listeners (i.e., listeners not
declared via @TestExecutionListeners) and listeners explicitly declared
via @TestExecutionListeners. Specifically, a NoClassDefFoundError would
cause a test to fail for an explicitly declared TestExecutionListener
but not for an implicitly declared one.

This commit addresses this issue by:

 - Always swallowing a NoClassDefFoundError for both implicitly and
   explicitly declared TestExecutionListeners.
 - Changing the log level from DEBUG to INFO to make such situations
   more visible to the average end user.

Issue: SPR-11347