Start with these relationships:
Products::__pkProductID = Product_Test::_fkProductID
Tests::__pkTestID = Product_Test::_fkTestID
You can place a portal to Product_Test on the Products layout to list and select Tests records for each given Products record. Fields from Tests can be included in the Portal to show additional info about each selected Tests record and the _fkTestID field can be set up with a value list for selecting Tests records by their ID field.