app: tests: Use DTS instead of PM

Unit test changes for DTS usage.

Signed-off-by: Markus Lassila <markus.lassila@nordicsemi.no>

Author: MarkusLassila

app/tests/at_commands/CMakeLists.txt

@@ -79,4 +79,4 @@ set(includes
   "${PROJECT_SOURCE_DIR}/../stubs"
 )
 
-target_include_directories(app PRIVATE ${includes})
+target_include_directories(app BEFORE PRIVATE ${includes})

app/tests/at_commands/include/zephyr/storage/flash_map.h

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2026 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+ */
+
+#ifndef ZEPHYR_STORAGE_FLASH_MAP_H__
+#define ZEPHYR_STORAGE_FLASH_MAP_H__
+
+/* Test stub: both MCUboot slots exist. */
+#define FIXED_PARTITION_EXISTS(label) 1
+
+/* Addresses are ignored by the TFM stubs, so any value is fine. */
+#define PARTITION_ADDRESS(label) 0
+
+#endif /* ZEPHYR_STORAGE_FLASH_MAP_H__ */