Fix testMetadataCachingLocalRepo

As content.xml.xz is now used, and then XZedSimpleMetadataRepository,
the cached file path differs and test needs to be updated.

Change-Id: I0b56179e371acf95aefbc34fb1256cb46d74227e
Reviewed-on: https://git.eclipse.org/r/c/equinox/rt.equinox.p2/+/188554
Tested-by: Equinox Bot <equinox-bot@eclipse.org>
Reviewed-by: Mickael Istria <mistria@redhat.com>
1 file changed