Reduce duplication between headless tests

[infomiho]

Based on https://github.com/wasp-lang/wasp/pull/2478#pullrequestreview-2860428020

We should investigate if it's possible to unify all the headless test configs and maybe some of the testing logic (for example, functions related to auth or email verification like https://github.com/wasp-lang/wasp/blob/main/waspc/examples/todoApp/headless-tests/tests/helpers.ts)