tests: add OpenTruncateRead test
tests: add OpenTruncateRead test

This is a regression test for the issue that was fixed by the
last commit.

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