SquishTests: Make sure that *UnderCursor() return a str

Without this, "line in lineUnderCursor(w)" can be False
although "line == lineUnderCursor(w)" is True.

Change-Id: I7fa6be7d237957706bf1ad93d7303c55b81f24ec
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

tests/system/shared/utils.py

@@ -110,7 +110,7 @@ def textUnderCursor(window, fromPos, toPos):
     cursor.movePosition(toPos, QTextCursor.KeepAnchor)
     returnValue = cursor.selectedText()
     cursor.setPosition(oldposition)
-    return returnValue
+    return str(returnValue)
 
 def which(program):
     # Don't use spawn.find_executable because it can't find .bat or