Test/demo data is already available in the SuiteWebapp project!
However, it is not obvious that it exists or how to install it even if you know it exists.
The feature is hidden in: ApplicationConfig > Apps management page.
The default view of the Apps management page is Card view.
I think that is what is making this feature hidden.
(I think there should have been an Import demo data checkbox on this warning message .)
Where as the List view is lot more clear, in that, it has the checkbox column Demo data loaded which clearly shows which module is installed with or without the demo data.
Even on editing the module there is no Import demo data option available. The checkbox is disabled even in edit mode.
Select the modules to install and check the checkbox Import demo data and click install button.
Note: There is a property called data.import.demo-data in the application.properties file which is set to false by default.
But even after setting it to true (in the %CATALINA_HOME%\webapps\axelor-erp\WEB-INF\classes folder of the tomcat and restart) the checkbox in the individual module edit window(Fig. A above) is still disabled.