Implement ungracful client disconnect into JAPI_TcpKeepAliveFunctionality unit test

[Michael-M-Baron]

A new feature was implemented in #49 using the TCP keep alive mechanism kernel built in to remove ungraceful disconnected client sockets (Due to network failure for example).

A unit test was added to test this feature with disconnecting clients. Implementing an ungraceful disconnect however is not trivial and was there skipped and moved to a separate issue (this issue here).

Currently the client socket is closed properly with close(). This has to be replaced with some code ungracefully killing the socket. May be mocking or some kind of that can be used for this purpose.

Lines of codes to be changed:
japi_test.cc