bitbake: toaster: fix incorrect file mode

Python 3 complains about 'wa' mode this way:
 ValueError: must have exactly one of create/read/write/append mode

Fixed by using 'a' mode.

[YOCTO #9584]

(Bitbake rev: eb23a84e079125a08d0c8e910f7035ad9584c432)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/toaster/toastergui/tests.py

@@ -304,7 +304,7 @@ class ViewTests(TestCase):
         """Download the recipe file generated for the custom image"""
 
         # Create a dummy recipe file for the custom image generation to read
-        open("/tmp/a_recipe.bb", 'wa').close()
+        open("/tmp/a_recipe.bb", 'a').close()
         response = self.client.get(reverse('customrecipedownload',
                                            args=(self.project.id,
                                                  self.customr.id)))