Not all targets respond to test_target_type

In https://github.com/fastlane/fastlane/pull/12212, @joshdholtz had to do the following because aggregate targets don't respond to `test_target_type?`:

```diff
- non_test_targets = targets.reject(&:test_target_type?)
+ non_test_targets = targets.reject do |t|
+ # Not all targets respond to `test_target_type?`
+ t.respond_to?(:test_target_type?) && t.test_target_type?
+ end
```

It would be nice if aggregate targets (and others?) would respond to `test_target_type?` by returning `false` instead of throwing an exception