Fix clean button locator in PF4 search. (#965)

As it was, we were getting an error when searching for the element,
getting False for is_displayed. Consequently, the clear button was
not clicked and a generic browser clear was called instead.
The text disappeared for a bit and then it got re-filled to the
search field.
It caused failures in tests such as
tests/foreman/ui/test_dashboard.py::test_positive_task_status
Changing type and locator fixes the issue.

airgun/widgets.py

@@ -752,7 +752,7 @@ class PF4Search(Search):
     ROOT = '//div[@class="foreman-search-bar"]'
     search_field = TextInput(locator=(".//input[@aria-label='Search input']"))
     search_button = Text(locator=(".//button[@aria-label='Search']"))
-    clear_button = Button(locator=(".//input[@aria-label='Reset search']"))
+    clear_button = Text(locator=(".//button[@aria-label='Reset search']"))
 
     def clear(self):
         """Clears search field value and re-trigger search to remove all