Add tests and oracles for Populate_spec

tests/spec/default_spec_combine.i

tests/spec/default_spec_combine.ml

tests/spec/default_spec_custom.i

tests/spec/default_spec_custom.ml

tests/spec/default_spec_mode.i

tests/spec/oracle/default_spec_combine.0.res.oracle

tests/spec/oracle/default_spec_combine.1.res.oracle

tests/spec/oracle/default_spec_combine.2.res.oracle

tests/spec/oracle/default_spec_custom.0.res.oracle

tests/spec/oracle/default_spec_custom.1.res.oracle

tests/spec/oracle/default_spec_custom.2.res.oracle

tests/spec/oracle/default_spec_mode.0.res.oracle

tests/spec/oracle/default_spec_mode.1.res.oracle

tests/spec/oracle/default_spec_mode.2.res.oracle

tests/spec/oracle/default_spec_mode.3.res.oracle

tests/spec/oracle/default_spec_mode.4.res.oracle

tests/spec/oracle/default_spec_mode.5.res.oracle

tests/spec/oracle/default_spec_mode.6.res.oracle

tests/spec/oracle/default_spec_mode.7.res.oracle