Yes we can modify existing test files with help of inheriting class and overwrite/override existing method.

Ex:If i want to alter one of test cases in sale module then i will create test file in my custom module and then inherit that class with help of import that class.

from odoo.addons.sale.tests.test_sale_order import TestSaleOrder 
@tagged('post_install', '-at_install')
class TestSaleOrderExtended(TestSaleOrder):
        //now simply write your logic here
        //You can call super to call main code and add your existing code OR you can overwrite method with your own logic

Hope it will Help
Thanks