fails on Linux

Changed test to properly report what is actually missing.

Change-Id: Id5d87d88c8b8da052405196cbd1c05ccb569b40d
Reviewed-on: https://git.eclipse.org/r/c/pde/eclipse.pde.ui/+/192083
Tested-by: PDE Bot <pde-bot@eclipse.org>
Reviewed-by: Andrey Loskutov <loskutov@gmx.de>