Add unit tests for DHCP

Three simple tests that test the basic functionality of the DHCP client.
They require that UDP checksums are off for now.

test/unit/dhcp/test_dhcp.h

@@ -0,0 +1,8 @@
+#ifndef __TEST_DHCP_H__
+#define __TEST_DHCP_H__
+
+#include "../lwip_check.h"
+
+Suite* dhcp_suite(void);
+
+#endif