[fuzz] advance alarm time to cover timer handlers (#3753)

tests/fuzz/ip6_send.cpp

@@ -26,6 +26,8 @@
  *  POSSIBILITY OF SUCH DAMAGE.
  */
 
+#define MAX_ITERATIONS 100
+
 #include <stddef.h>
 
 #include <openthread/instance.h>
@@ -39,6 +41,7 @@
 #include "common/code_utils.hpp"
 
 extern "C" void FuzzerPlatformInit(void);
+extern "C" void FuzzerPlatformProcess(otInstance *aInstance);
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 {
@@ -72,9 +75,14 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 
     message = NULL;
 
-    while (otTaskletsArePending(instance))
+    for (int i = 0; i < MAX_ITERATIONS; i++)
     {
-        otTaskletsProcess(instance);
+        while (otTaskletsArePending(instance))
+        {
+            otTaskletsProcess(instance);
+        }
+
+        FuzzerPlatformProcess(instance);
     }
 
 exit: