Demonstration data is the most important data in odoo-9 so it's possible to test it easily. This is done by adding a demodata file, which must be linked from the manifest. For example you can see below code .

<openerp>
    <data>
        <record id="padilla" model="academy.teachers">
            <field name="name">Diana Padilla</field>
        </record>
        <record id="carroll" model="academy.teachers">
            <field name="name">Jody Carroll</field>
        </record>
        <record id="vaughn" model="academy.teachers">
            <field name="name">Lester Vaughn</field>
        </record>
    </data>
</openerp>

Note- Data files can be used for demo and non-demo data. Demo data are only loaded in "demonstration mode"