stupidgcm: add test for in-place Open
stupidgcm: add test for in-place Open

Adds a test for the optimization introduced in:

	stupidgcm: Open: if "dst" is big enough, use it as the output buffer

rfjakob/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs,davesteele/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs