tests: poison $HOME and $XDG_RUNTIME_DIR env variables

tests: poison $HOME and $XDG_RUNTIME_DIR env variables

To attempt to catch unit tests which accidentally create files in $HOME,
or $XDG_RUNTIME_DIR, poison these env vars by pointing them to
directories which don't exist. This should give easier to debug test
failures. For example:

$ VIR_TEST_DEBUG=1 ./qemuhotplugtest
Could not initialize HostdevManager - operation failed: Failed to create state dir '/bad-test-used-env-xdg-runtime-dir/libvirt/hostdevmgr'

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>