Do not run NTP Search Box experiment (#3537)

Task/Issue URL:
https://app.asana.com/0/1201037661562251/1208734291836545/f
Tech Design URL:
CC:

**Description**:
Avoid showing the NTP Search Box when running UI Tests.

**Definition of Done**:

* [x] Does this PR satisfy our [Definition of
Done](https://app.asana.com/0/1202500774821704/1207634633537039/f)?

---
###### Internal references:
[Pull Request Review
Checklist](https://app.asana.com/0/1202500774821704/1203764234894239/f)
[Software Engineering
Expectations](https://app.asana.com/0/59792373528535/199064865822552)
[Technical Design
Template](https://app.asana.com/0/59792373528535/184709971311943)
[Pull Request
Documentation](https://app.asana.com/0/1202500774821704/1204012835277482/f)

DuckDuckGo/HomePage/Model/NewTabPageSearchBoxExperiment.swift

@@ -75,6 +75,9 @@ protocol NewTabPageSearchBoxExperimentCohortDeciding {
 
 struct DefaultNewTabPageSearchBoxExperimentCohortDecider: NewTabPageSearchBoxExperimentCohortDeciding {
     var cohort: NewTabPageSearchBoxExperiment.Cohort? {
+        if NSApp.runType == .uiTests {
+            return nil
+        }
 
         // We enroll all new users
         if AppDelegate.isNewUser {