Update TestSystemPacketBuffer (#4305)

* Update TestSystemPacketBuffer

#### Problem

Uses of `PacketBuffer` have largely been replaced by
`PacketBufferHandle`. The unit test had not kept up.

#### Summary of Changes

- Added tests for PacketBufferHandle specifically.
- Added many checks of reference counts to verify the ownership model.
- Some more refactoring into `PacketBufferTest`, friend class of
  `PacketBuffer` and `PacketBufferHandle`, to use private fields
  and track buffer allocations.

part of #2707 - Figure out a way to express PacketBuffer ownership in the type system

* Release buffers more often so that tests work with small pools.

src/system/SystemPacketBuffer.cpp

@@ -1,6 +1,6 @@
 /*
  *
- *    Copyright (c) 2020 Project CHIP Authors
+ *    Copyright (c) 2020-2021 Project CHIP Authors
  *    Copyright (c) 2016-2017 Nest Labs, Inc.
  *
  *    Licensed under the Apache License, Version 2.0 (the "License");
@@ -158,8 +158,11 @@ uint16_t PacketBuffer::ReservedSize() const
     return static_cast<uint16_t>(kDelta - CHIP_SYSTEM_PACKETBUFFER_HEADER_SIZE);
 }
 
-void PacketBuffer::AddToEnd_ForNow(PacketBuffer * aPacket)
+void PacketBuffer::AddToEnd(PacketBufferHandle aPacketHandle)
 {
+    PacketBuffer * aPacket = aPacketHandle.mBuffer;
+    aPacketHandle.mBuffer  = nullptr;
+
 #if CHIP_SYSTEM_CONFIG_USE_LWIP
     pbuf_cat(this, aPacket);
 #else  // !CHIP_SYSTEM_CONFIG_USE_LWIP
@@ -179,11 +182,6 @@ void PacketBuffer::AddToEnd_ForNow(PacketBuffer * aPacket)
 #endif // !CHIP_SYSTEM_CONFIG_USE_LWIP
 }
 
-void PacketBuffer::AddToEnd(PacketBufferHandle aPacketHandle)
-{
-    AddToEnd_ForNow(aPacketHandle.Release_ForNow());
-}
-
 void PacketBuffer::CompactHead()
 {
     uint8_t * const kStart = reinterpret_cast<uint8_t *>(this) + CHIP_SYSTEM_PACKETBUFFER_HEADER_SIZE;

src/system/SystemPacketBuffer.h

@@ -1,6 +1,6 @@
 /*
  *
- *    Copyright (c) 2020 Project CHIP Authors
+ *    Copyright (c) 2020-2021 Project CHIP Authors
  *    Copyright (c) 2016-2017 Nest Labs, Inc.
  *
  *    Licensed under the Apache License, Version 2.0 (the "License");
@@ -254,7 +254,7 @@ class DLL_EXPORT PacketBuffer : private pbuf
     /**
      * Return the last buffer in a buffer chain.
      *
-     *  @return a handle to the next buffer in the buffer chain.
+     *  @return a handle to the last buffer in the buffer chain.
      */
     CHECK_RETURN_VALUE PacketBufferHandle Last();
 
@@ -328,7 +328,6 @@ class DLL_EXPORT PacketBuffer : private pbuf
     PacketBuffer * ChainedBuffer() const { return static_cast<PacketBuffer *>(this->next); }
     PacketBuffer * Consume(uint16_t aConsumeLength);
     void Clear();
-    void AddToEnd_ForNow(PacketBuffer * aPacket);
 
     friend class PacketBufferHandle;
     friend class ::PacketBufferTest;
@@ -623,6 +622,8 @@ class DLL_EXPORT PacketBufferHandle
 
     PacketBuffer * Get() const { return mBuffer; }
 
+    bool operator==(const PacketBufferHandle & aOther) { return mBuffer == aOther.mBuffer; }
+
     PacketBuffer * mBuffer;
 
     friend class PacketBuffer;