[nexus] configure parent search backoff interval (openthread#12993)

Added OPENTHREAD_CONFIG_PARENT_SEARCH_BACKOFF_INTERVAL with a value of
10 minutes (10 * 60 seconds) to the Nexus core configuration. This
helps in controlling the backoff behavior during parent search in the
simulator, making it more interactive.

Author: jwhui

tests/nexus/openthread-core-nexus-config.h

@@ -127,6 +127,7 @@
 #define OPENTHREAD_CONFIG_NET_DIAG_VENDOR_SW_VERSION "OT-simul-nexus"
 #define OPENTHREAD_CONFIG_NETDATA_PUBLISHER_ENABLE 1
 #define OPENTHREAD_CONFIG_NUM_MESSAGE_BUFFERS 256
+#define OPENTHREAD_CONFIG_PARENT_SEARCH_BACKOFF_INTERVAL (10 * 60)
 #define OPENTHREAD_CONFIG_PLATFORM_DNS_ENABLE 1
 #define OPENTHREAD_CONFIG_PLATFORM_DNSSD_ALLOW_RUN_TIME_SELECTION 0
 #define OPENTHREAD_CONFIG_PLATFORM_DNSSD_ENABLE 0