Changes

tests/hwloc_backends: make sure synthetic/xml are not forced loaded
Clear synthetic and xml env vars so that we don't force load them 
instead of falling back to defaults components
It failed on Windows because HWLOC_COMPONENTS being set to empty means
unset, causing HWLOC_SYNTHETIC and HWLOC_XMLFILE to be processed.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

tests/hwloc_backends: fix on non-x86
Blacklisting the x86 component on non-x86 hardware must fail.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>