Fix tests broken by rB58632a7f3c0f1be6.

Commits breaking RNA API should always run all tests, and do text search
in python code base to ensure everything is updated as needed.

tests/python/bl_blendfile_io.py

@@ -73,7 +73,7 @@ def main():
     args = argparse_create().parse_args()
 
     # Don't write thumbnails into the home directory.
-    bpy.context.preferences.filepaths.use_save_preview_images = False
+    bpy.context.preferences.filepaths.file_preview_type = 'NONE'
 
     for Test in TESTS:
         Test(args).run_all_tests()

tests/python/bl_blendfile_liblink.py

@@ -278,7 +278,7 @@ def main():
     args = argparse_create().parse_args()
 
     # Don't write thumbnails into the home directory.
-    bpy.context.preferences.filepaths.use_save_preview_images = False
+    bpy.context.preferences.filepaths.file_preview_type = 'NONE'
 
     for Test in TESTS:
         Test(args).run_all_tests()

tests/python/bl_blendfile_library_overrides.py

@@ -208,7 +208,7 @@ def main():
     args = argparse_create().parse_args()
 
     # Don't write thumbnails into the home directory.
-    bpy.context.preferences.filepaths.use_save_preview_images = False
+    bpy.context.preferences.filepaths.file_preview_type = 'NONE'
     bpy.context.preferences.experimental.use_override_templates = True
 
     for Test in TESTS: