Motivation:
We have a few code paths where `Chunk.remainingCapacity()` is called
without regard for exclusive access to the given chunk. The `BuddyChunk`
implementation unfortunately drained the `freeList` in this call,
leading to racy modifications of the `buddies` array, that in turn can
manifest as unpredictable bugs.

Modification:
Make the `BuddyChunk.remainingCapacity` non-modifying. Instead of
draining the `freeList`, we sum up the capacity held by it and add it to
the capacity accounted for in the `Chunk` super class.

This operation needs a `weakPeekReduce` method on the `MpscIntQueue`.
This method performs a reduction operation on a given range of entries,
but without removing entries or incrementing the consumer index.

A regression test is added, which successfully captured the issue before
it was fixed.

Also did a few test cleanups.

Result:
No more corruptions in the buddies array.

This fixes the rare occurrences of assertion errors like the following:

```
java.lang.AssertionError
	at io.netty.buffer.AdaptivePoolingAllocator$Magazine.allocate(AdaptivePoolingAllocator.java:916)
	at io.netty.buffer.AdaptivePoolingAllocator$Magazine.tryAllocate(AdaptivePoolingAllocator.java:854)
	at io.netty.buffer.AdaptivePoolingAllocator$MagazineGroup.allocate(AdaptivePoolingAllocator.java:421)
	at io.netty.buffer.AdaptivePoolingAllocator.allocate(AdaptivePoolingAllocator.java:271)
```

(cherry picked from commit bd866c3)