[Libguestfs] [PATCH] build: Enable parallel tests.

This patch enables parallel tests. This has a small performance benefit. Unfortunately several tests still overwrite common filenames (eg. 'test.out') which means that if you apply this patch then tests will fail randomly. To fix this requires every test to be examined manually to ensure that it does not use a common filename, eg. changing 'test.out' to '<testname>.out'. Rich.