tests: Split regressions -> various subdirectories of tests/
Fix regressions/rhbz557655.sh when debugging is enabled (v2). The previous fix didn't cope with ordinary warnings emitted by qemu (eg. "open /dev/kvm: No such file or directory"). This is a hopefully more complete fix for the problem. See also commit 3cd7ce75f1ce5048a4d9f6aeaf66aff3194e1096.