tests: drivers: flash: stm32: handle conflict for storage partition

Since the addition of a new storage partition definition in
the nucleo_f746zg dts, this node must be deleted before performing
this test.

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>

Author: djiatsaf-st

tests/drivers/flash/stm32/boards/nucleo_f746zg.overlay

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+/delete-node/ &storage_partition;
 &flash0 {
 	partitions {
 		compatible = "fixed-partitions";