Testsuite / PAC: use sparse representation for the model

tests/pac/trend-component-14/example1.mod

tests/pac/trend-component-14/substitution.mod

tests/pac/var-5/example1.mod

tests/pac/var-5/substitution.mod

tests/pac/var-6/example1.mod

tests/pac/var-6/substitution.mod

tests/pac/var-7/example1.mod

tests/pac/var-7/substitution.mod

tests/pac/var-8/example1.mod

tests/pac/var-8/substitution.mod

tests/pac/var-9/example1.mod

tests/pac/var-9/substitution.mod