[gnome-builder] build: Define test_sources, don't append

[Christian Hergert <chergert src gnome org>]

commit 38d2e67318374f09f74573982ca096f43662e2e8
Author: Matt Turner <mattst88 gmail com>
Date:   Mon May 16 18:06:24 2022 -0400

    build: Define test_sources, don't append
    
    If building without git support, meson setup fails with
    
    src/plugins/meson/meson.build:30:0: ERROR: Unknown variable "test_sources".
    
    I believe the += a mistake in commit c2e10cdec ("meson: fixed language
    parsing in meson files") which wasn't noticed because there was an
    existing test_sources variable (in the git plugin's meson.build, see
    next commit).
    
    Bug: https://bugs.gentoo.org/844103

 src/plugins/meson/meson.build | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/plugins/meson/meson.build b/src/plugins/meson/meson.build
index f5161cc07..38923ed21 100644
--- a/src/plugins/meson/meson.build
+++ b/src/plugins/meson/meson.build
@@ -27,7 +27,7 @@ plugin_meson_resources = gnome.compile_resources(
 
 plugins_sources += plugin_meson_resources
 
-test_sources += files([
+test_sources = files([
   'test-meson-build-system.c',
   'gbp-meson-build-system.c',
   'gbp-meson-toolchain.c',