tests: avoid spurious failure of nodeinfotest

tests: avoid spurious failure of nodeinfotest

When running 'make check' under a multi-cpu Dom0 xen machine,
nodeinfotest had a spurious failure it was reading from
/sys/devices/system/cpu, but xen has no notion of topology.  The test
was intended to be isolated from reading any real system files; the
regression was introduced in Mar 2010 with commit aa2f6f96dd.

Fix things by allowing an early exit for the testsuite.

* src/nodeinfo.c (linuxNodeInfoCPUPopulate): Add parameter.
(nodeGetInfo): Adjust caller.
* tests/nodeinfotest.c (linuxTestCompareFiles): Likewise.