tests: add loopback-mem script
tests: add loopback-mem script

Helps to find memory leaks in go-fuse's loopback, which
gocryptfs builds upon.

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