Don't write to user datadir in test_qt_javascript.

This was more complicated than the other data/config/cachedir test
fixes, as QtWebEngine was accessing the datadir directly (and bypassing
standdarddir.data).

This means the tmpdir_data stub is not enough, we need to set
XDG_DATA_HOME to redirect access.

tests/unit/browser/webkit/test_qt_javascript.py

@@ -21,6 +21,7 @@
 
 
 import pytest
+import os
 
 from PyQt5.QtCore import QObject, pyqtSignal
 from PyQt5.QtWebKit import QWebSettings
@@ -65,12 +66,17 @@ def test_element_js_webkit(webview, js_enabled, expected):
 
 
 @pytest.mark.parametrize('js_enabled, expected', [(True, 2.0), (False, 2.0)])
-def test_simple_js_webengine(qtbot, webengineview, js_enabled, expected):
+def test_simple_js_webengine(qtbot, webengineview, js_enabled, expected,
+                             data_tmpdir):
     """With QtWebEngine, runJavaScript works even when JS is off."""
     # pylint: disable=no-name-in-module,useless-suppression
     # If we get there (because of the webengineview fixture) we can be certain
     # QtWebEngine is available
     from PyQt5.QtWebEngineWidgets import QWebEngineSettings
+    # runJavaScript will write to the data directory. As the path is determined
+    # by Qt and bypasses standarddir.data(), the data_tmpdir fixture is not
+    # enough -- we need to set XDG_DATA_HOME as well
+    os.putenv('XDG_DATA_HOME', str(data_tmpdir))
     webengineview.settings().setAttribute(QWebEngineSettings.JavascriptEnabled,
                                           js_enabled)