[fuzz] process pending tasklets to increase coverage (#3617)

tests/fuzz/ip6_send.cpp

@@ -32,6 +32,7 @@
 #include <openthread/ip6.h>
 #include <openthread/link.h>
 #include <openthread/message.h>
+#include <openthread/tasklet.h>
 #include <openthread/thread.h>
 #include <openthread/thread_ftd.h>
 
@@ -71,6 +72,11 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 
     message = NULL;
 
+    while (otTaskletsArePending(instance))
+    {
+        otTaskletsProcess(instance);
+    }
+
 exit:
 
     if (message != NULL)