Fix overriden method not called in TrafficShaping

Motivation:
handlerAdded and handlerRemoved were overriden but super was never
called, while it should.
Also add one missing information in the toString method.

Modifications:
Add the super corresponding call, and add checkInterval to the
toString() method

Result;
super method calls are correctly passed to the super implementation
part.
Improve error reporting in SingleThreadEventLoopTest. Fixes #2894

Motivation:

When an assertTrue(condition) statement fails we usually don't know
why, as the parameters of the condition are not logged.

Modifications:

Include relevant parameters in the assertion error message.

Result:

Easier to debug and understand test failures.