Xenstore testsuite robustness: save output rather than rerun on failure.
"make check" reruns a test which fails with more verbosity. If the test
fails intermittently, that doesn't work well: save the output and simply
dump it if the test fails.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>