tests: Stop using native_posix/sim transitional headers

As native_posix has been removed, these tests do not need to support
anymore both native_sim and _posix targets, so we can stop using
the transitional headers, and APIs.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

tests/boards/native_sim/rtc/src/main.c

diff --git a/tests/boards/native_sim/rtc/src/main.c b/tests/boards/native_sim/rtc/src/main.c
index fdcdc37..2617781 100644
--- a/tests/boards/native_sim/rtc/src/main.c
+++ b/tests/boards/native_sim/rtc/src/main.c
@@ -12,7 +12,8 @@
 #include <stdio.h>
 #include <time.h>
 
-#include "timer_model.h"
+#include <nsi_hw_scheduler.h>
+#include <nsi_timer_model.h>
 #include "native_rtc.h"
 
 #include <stdio.h>

tests/net/lib/coap_client/src/main.c

diff --git a/tests/net/lib/coap_client/src/main.c b/tests/net/lib/coap_client/src/main.c
index 02d7693..5e3ea4b 100644
--- a/tests/net/lib/coap_client/src/main.c
+++ b/tests/net/lib/coap_client/src/main.c
@@ -9,7 +9,7 @@
 #include <zephyr/misc/lorem_ipsum.h>
 #include <zephyr/ztest.h>
 #if defined(CONFIG_NATIVE_SIM_SLOWDOWN_TO_REAL_TIME)
-#include "timer_model.h"
+#include "nsi_timer_model.h"
 #endif
 #include "stubs.h"
 

tests/net/lib/lwm2m/lwm2m_engine/src/main.c

diff --git a/tests/net/lib/lwm2m/lwm2m_engine/src/main.c b/tests/net/lib/lwm2m/lwm2m_engine/src/main.c
index fc1f8de..9331eae 100644
--- a/tests/net/lib/lwm2m/lwm2m_engine/src/main.c
+++ b/tests/net/lib/lwm2m/lwm2m_engine/src/main.c
@@ -14,7 +14,7 @@
 
 #include "stubs.h"
 #if defined(CONFIG_NATIVE_SIM_SLOWDOWN_TO_REAL_TIME)
-#include "timer_model.h"
+#include "nsi_timer_model.h"
 #endif
 
 #define LOG_LEVEL	LOG_LEVEL_DBG