TMF: Renamed o.e.l.tmf.analysis.xml.core.tests stubs folder to common

TMF: Renamed o.e.l.tmf.analysis.xml.core.tests stubs folder to common

As mentioned as a last comment in the https://git.eclipse.org/r/#/c/19914/
patch, this folder contains test traces and not implementation stubs so
the name is more fit as "common".

Change-Id: Ia274cb4de2086a0311ffea4a1b95f6407d584b77
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/21294
Tested-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
IP-Clean: Matthew Khouzam <matthew.khouzam@ericsson.com>