basesrc: preserve seqnum of eos events sent by the user

basesrc: preserve seqnum of eos events sent by the user

Store the eos event seqnum and use it when creating the
new eos event to be pushed downstream. To know if the eos
was caused by the eos events received on send_event, a
'forced_eos' flag is used to use the correct seqnum on
the event pushed downstream.

Useful if the application wants to check if the EOS message
was generated from its own pushed EOS or from another source
(stream really finished).

Also adds a test for this

https://bugzilla.gnome.org/show_bug.cgi?id=722791