When doing test-driven development, once you've written a failing (a.k.a. red) test, the main objective is to get to a green (a.k.a. passing) test as quickly as possible via the simplest method.
Even if that means returning a hard-coded value.
Once the test passes, it or the code it's testing can be refactored.
Or you can move on to the next test and, if you need to change the hard-coded value to get both tests to pass, then it is the right time to do so.
P.S. There's less than a year until Drupal 7's end-of-life date. Plan your upgrade to Drupal 10 now!