TEST_DRIVER_DIR is defined as "$(top_builddir)/src/.libs"; however,
as of commit bc6e206322ae, virDriverLoadModule() will search (the
absolute version of) that directory automatically, which means
passing it through the environment is no longer necessary.
Signed-off-by: Andrea Bolognani <abologna@redhat.com> Reviewed-by: Martin Kletzander <mkletzan@redhat.com>