gnrc_sixlowpan_frag: encapsulate msg_send_to_self()

This makes it easier to access this functionality for test cases e.g.
from the `main` thread.

sys/include/net/gnrc/sixlowpan/frag.h

@@ -29,9 +29,10 @@
 #include <stdbool.h>
 
 #include "byteorder.h"
-#include "kernel_types.h"
+#include "msg.h"
 #include "net/gnrc/pkt.h"
 #include "net/gnrc/netif/hdr.h"
+#include "net/gnrc/sixlowpan/internal.h"
 #include "net/ieee802154.h"
 #include "net/sixlowpan.h"
 
@@ -133,6 +134,29 @@ void gnrc_sixlowpan_frag_recv(gnrc_pktsnip_t *pkt, void *ctx, unsigned page);
  */
 void gnrc_sixlowpan_frag_rbuf_gc(void);
 
+/**
+ * @brief   Sends a message to pass a further fragment down the network stack
+ *
+ * @see GNRC_SIXLOWPAN_MSG_FRAG_SND
+ *
+ * @param[in] fragment_msg  A @ref gnrc_sixlowpan_msg_frag_t object
+ *
+ * @return  true, when the message was sent
+ * @return  false when sending the message failed.
+ */
+static inline bool gnrc_sixlowpan_frag_send_msg(gnrc_sixlowpan_msg_frag_t *fragment_msg)
+{
+    msg_t msg;
+
+    msg.content.ptr = fragment_msg;
+    msg.type = GNRC_SIXLOWPAN_MSG_FRAG_SND;
+#ifdef TEST_SUITES
+    return (msg_try_send(&msg, gnrc_sixlowpan_get_pid()) > 0);
+#else
+    return (msg_send_to_self(&msg) != 0);
+#endif
+}
+
 #if defined(MODULE_GNRC_SIXLOWPAN_FRAG) || defined(DOXYGEN)
 /**
  * @brief   Removes an entry from the reassembly buffer

sys/include/net/gnrc/sixlowpan/internal.h

@@ -27,6 +27,14 @@
 extern "C" {
 #endif
 
+/**
+ * @brief   Returns the PID of the 6Lo thread
+ *
+ * @return  The PID of the 6Lo thread on success
+ * @return  KERNEL_PID_UNDEF, when 6Lo thread was not started.
+ */
+kernel_pid_t gnrc_sixlowpan_get_pid(void);
+
 /**
  * @brief   Delegates a packet to the network layer
  *

sys/net/gnrc/network_layer/sixlowpan/frag/gnrc_sixlowpan_frag.c

@@ -241,7 +241,6 @@ void gnrc_sixlowpan_frag_send(gnrc_pktsnip_t *pkt, void *ctx, unsigned page)
     /* payload_len: actual size of the packet vs
      * datagram_size: size of the uncompressed IPv6 packet */
     size_t payload_len = gnrc_pkt_len(fragment_msg->pkt->next);
-    msg_t msg;
 
     assert((fragment_msg->pkt == pkt) || (pkt == NULL));
     (void)page;
@@ -284,9 +283,7 @@ void gnrc_sixlowpan_frag_send(gnrc_pktsnip_t *pkt, void *ctx, unsigned page)
         goto error;
     }
     fragment_msg->offset += res;
-    msg.type = GNRC_SIXLOWPAN_MSG_FRAG_SND,
-    msg.content.ptr = fragment_msg;
-    if (msg_send_to_self(&msg) == 0) {
+    if (!gnrc_sixlowpan_frag_send_msg(fragment_msg)) {
         DEBUG("6lo frag: message queue full, can't issue next fragment "
               "sending\n");
         goto error;

sys/net/gnrc/network_layer/sixlowpan/gnrc_sixlowpan.c

@@ -55,6 +55,11 @@ kernel_pid_t gnrc_sixlowpan_init(void)
     return _pid;
 }
 
+kernel_pid_t gnrc_sixlowpan_get_pid(void)
+{
+    return _pid;
+}
+
 void gnrc_sixlowpan_dispatch_recv(gnrc_pktsnip_t *pkt, void *context,
                                   unsigned page)
 {