Add SPDX license headers to multiple source files

Refs #1085 and closes #1099

test_apps/main/test_common.c

@@ -1,3 +1,9 @@
+/*
+ * SPDX-FileCopyrightText: 2025 Kozmotronik Tech
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
 #include "test_common.h"
 #include "relay_chn_ctl.h" // For resetting the channels
 #include "relay_chn_tilt.h" // For resetting tilt count