[tests] prevent settings/flash retaining between unit tests (#12875)

Issue: state was retained between OT instances in the unit test platform, across tests.
This commit adds settings and flash clearing as part of testInitInstance().

tests/unit/test_platform.cpp

@@ -33,6 +33,8 @@
 
 #include <stdio.h>
 #include <sys/time.h>
+#include <openthread/platform/flash.h>
+
 #ifdef OPENTHREAD_CONFIG_BLE_TCAT_ENABLE
 #include <openthread/tcat.h>
 #include <openthread/platform/ble.h>
@@ -50,6 +52,12 @@ ot::Instance *testInitInstance(void)
 {
     otInstance *instance = nullptr;
 
+    settings.clear();
+    for (uint8_t idx = 0; idx < FLASH_SWAP_NUM; idx++)
+    {
+        otPlatFlashErase(nullptr, idx);
+    }
+
 #if OPENTHREAD_CONFIG_MULTIPLE_INSTANCE_ENABLE
 #if OPENTHREAD_CONFIG_MULTIPLE_STATIC_INSTANCE_ENABLE
     instance = otInstanceInitMultiple(0);