tests/build: Silence meson deprecation warning

This produced the following warning:
```
DEPRECATION: Project uses feature that was always broken, and is now
deprecated since '1.3.0': str.format: Value other than strings,
integers, bools, options, dictionaries and lists thereof..
```

The resulting value of `SHELL_BACKGROUND_IMAGE` was indeed bogus, not
containing a proper file path.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3123>

diff --git a/tests/meson.build b/tests/meson.build
index 821c83cd93fdcbbff0e8b83083bed8581df77b91..da4d40f0503b78bc0c5cee7930f53fab2dd72f12 100644 (file)
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -56,7 +56,7 @@ shell_tests = [
 gvc_typelib_path = fs.parent(libgvc.get_variable('libgvc_gir')[1].full_path())
 libgvc_path = fs.parent(libgvc.get_variable('libgvc').full_path())
 
-background_file = files(join_paths('data', 'background.png'))
+background_file = join_paths(meson.current_source_dir(), 'data', 'background.png')
 
 shell_testenv = environment()
 shell_testenv.set('G_DEBUG', 'fatal-warnings')