SquishTests: Fix writing test files on Windows

Writing the text test files without encoding can end up in
ANSI encoding of the files and in turn a fail when opening
as the file is treated read only until correct encoding is
used.

Change-Id: I9e21e240b3b02011a2266dfda1f1b453c3dad457
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

tests/system/suite_editors/tst_clean_whitespaces/test.py

@@ -89,7 +89,7 @@ def prepareFileExternal(fileName, content):
                 test.log("Added trailing whitespace.")
         modifiedContent += currentLine
 
-    with open(fileName, "w") as f:
+    with open(fileName, "w", encoding="utf-8") as f: # used only with text files, so okay
         f.write(modifiedContent)
 
     if not emptyLine or not trailingWS: