Remove calls to drupal_valid_test_ua() from production code

Hi @phenaproxima,
That makes sense—mixing test-related logic into production code with drupal_valid_test_ua() can make the codebase harder to maintain.
So I'm planning to Introduce a service to determine test mode (TestModeChecker), and will replace drupal_valid_test_ua() calls with service usage.

Would it be a good solution?

I don't think that would really solve the problem, to be honest; the idea is to remove any testing-related code from the production code. There are other approaches we can take, but they'll vary based on the specific use. In other words, we're going to need a few different approaches for this one.

I'll take this on!