Make all ChunkStore impls crash on Write() after Close/Ref()

I incorrectly said this was already true in a previous patch, so
make it true now!

Also, updated file_store_test.go and s3_store_test.go to use stretchr
testify suites so they can share setup and teardown code.

Fixes issue #5 as well

chunks/memory_store.go

@@ -65,8 +65,6 @@ func (w *memoryChunkWriter) Ref() (ref.Ref, error) {
 }
 
 func (w *memoryChunkWriter) Close() error {
-	// Not really necessary, but this will at least free memory and cause subsequent operations to crash.
-	// BUG 17: Make this method consistent with other ChunkStore implementations.
 	*w = memoryChunkWriter{}
 	return nil
 }