tmf: Re-enable the unit tests depending on CTF traces

tmf: Re-enable the unit tests depending on CTF traces

And skip them if the traces are not available.

Also added a new exported test file manager, so that other
plugin tests (like the lttng2 ones) can re-use it.

Change-Id: I71cffad8f68089f3e88582e436a74bb2ec57c897
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/10354
Reviewed-by: Bernd Hufmann <bhufmann@gmail.com>
IP-Clean: Bernd Hufmann <bhufmann@gmail.com>
Tested-by: Bernd Hufmann <bhufmann@gmail.com>
Tested-by: Hudson CI