webui: make installer.reach method more robust

So that is is able to handle the case where it starts from the page
whose next page has also some other possible previous page.

ui/webui/test/helpers/installer.py

@@ -60,13 +60,13 @@ def begin_installation(self, should_fail=False, confirm_erase=True):
 
     def reach(self, target_page):
         path = []
-        page = target_page
+        prev_pages = [target_page]
         current_page = self.get_current_page()
 
-        while current_page != page:
+        while current_page not in prev_pages:
+            page = prev_pages[0]
             path.append(page)
-            prev = [k for k, v in self.steps._steps_jump.items() if page in v][0]
-            page = prev
+            prev_pages = [k for k, v in self.steps._steps_jump.items() if page in v]
 
         while self.get_current_page() != target_page:
             next_page = path.pop()