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

tests/fuzz/cli_uart_received.cpp

@@ -33,6 +33,7 @@
 #include <openthread/instance.h>
 #include <openthread/ip6.h>
 #include <openthread/link.h>
+#include <openthread/tasklet.h>
 #include <openthread/thread.h>
 #include <openthread/thread_ftd.h>
 #include <openthread/platform/uart.h>
@@ -65,6 +66,11 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 
     otPlatUartReceived(buf, (uint16_t)size);
 
+    while (otTaskletsArePending(instance))
+    {
+        otTaskletsProcess(instance);
+    }
+
 exit:
 
     if (buf != NULL)