test_build.sh: use -C option of ninja to specify the build directory

Change-Id: I04a0fdf9b5126b9f4006e8229c3926ceb1013456
Signed-off-by: Thomas Heijligen <thomas.heijligen@secunet.de>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/51979
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/test_build.sh b/test_build.sh
index 8f11ccd..dc286f8 100755
--- a/test_build.sh
+++ b/test_build.sh
@@ -3,6 +3,8 @@
 
 make CONFIG_EVERYTHING=yes WARNERROR=yes
 
-meson out
-(cd out && ninja)
-(cd out && ninja test)
+
+builddir=out
+meson $builddir
+ninja -C $builddir
+ninja -C $builddir test