[tests] oracles: empty struct refused by default

tests/crowbar/complete_type.ml

tests/crowbar/mutable.ml

tests/crowbar/offset_anonymous_field.ml

tests/rte/initialized_union.c

tests/rte/oracle/initialized_union.0.res.oracle

tests/rte/oracle/initialized_union.1.res.oracle

tests/value/empty_struct2.c

tests/value/oracle/empty_base.1.res.oracle

tests/value/oracle/empty_struct.3.res.oracle

tests/value/oracle/empty_struct.4.res.oracle

tests/value/oracle/empty_struct2.res.oracle