tests: Fix test assertion with umockdev

Since 0.9.4, rather than checking for libumockdev-preload being in
LD_PRELOAD, umockdev_in_mock_environment now checks whether /sys is
being redirected. This only occurs when $UMOCKDEV_DIR is set, which
happens inside umockdev_testbed_new, so the assertion must come after
this call.

https://bugzilla.gnome.org/show_bug.cgi?id=792845