tmf: Fix TmfTestTrace.getFullPath()

tmf: Fix TmfTestTrace.getFullPath()

Change getFullPath() to return an absolute path and update callers
accordingly. The internal directory is no longer a relative path from
the plug-in installation location, which allows getTrace() to work
properly when called from another plug-in.

Change-Id: I761ce633da8a64e33fe83e062478da566d5c795c
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/91408
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>