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!

This doesn't go as far as I would like, but trying to completely remove switches that are only used for testing turned out to be a little bit dragon-shaped. I think we can have follow-ups to properly do the remainder of the cleanup (i.e., making certain things actual configuration options), but this at least gets rid of the ugly calls to drupal_valid_test_ua().

Tagging for backport to 2.0.x.

This looks good. I remember feeling a way about having `drupal_valid_test_ua()` in runtime code when we added it, glad to see it gone.

rebased and merged onto 2.1.x, conflicts in backport

backported! thanks, @phenaproxima

Automatically closed - issue fixed for 2 weeks with no activity.