tmf: Decouple tmf.core unit tests from TmfEvent

tmf: Decouple tmf.core unit tests from TmfEvent

Make sure the TMF unit tests use the ITmfEvent interface
as much as possible (this is the behavior we want to have
in the core framework).

Change-Id: I242a5265c716498392f2b1414785db03f302540d
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/10051
Tested-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
IP-Clean: Patrick Tasse <patrick.tasse@gmail.com>