[pygobject] tests: force Adwaita as gtk theme

[Christoph Reiter <creiter src gnome org>]

commit 492dd2d24470faf4044ac560747aa9783ec2c7a6
Author: Christoph Reiter <reiter christoph gmail com>
Date:   Sun Aug 19 15:08:54 2018 +0200

    tests: force Adwaita as gtk theme
    
    The Ubuntu 18.10 theme has css errors which make the gtk tests
    fail.

 tests/__init__.py | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/tests/__init__.py b/tests/__init__.py
index 1611db33..1ab73f0d 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -89,6 +89,9 @@ def init_test_environ():
     os.environ['GSETTINGS_SCHEMA_DIR'] = tests_builddir
     os.environ['G_FILENAME_ENCODING'] = 'UTF-8'
 
+    # Force the default theme so broken themes don't affect the tests
+    os.environ['GTK_THEME'] = 'Adwaita'
+
     import gi
     gi.require_version("GIRepository", "2.0")
     from gi.repository import GIRepository