tests: add TestXfs124
tests: add TestXfs124

This test reproduces the problem xfstests generic/124 uncovered.
The warning itself is harmless, but we should either (1) add locking
so that this cannot happen anymore or (2) drop the warning.

Currently fails:

$ go test -v
=== RUN   Test1980Tar
--- PASS: Test1980Tar (0.00s)
=== RUN   TestCtlSock
--- PASS: TestCtlSock (0.10s)
=== RUN   TestOpenTruncateRead
--- PASS: TestOpenTruncateRead (0.00s)
=== RUN   TestWORead
--- PASS: TestWORead (0.00s)
=== RUN   TestXfs124
cipherSize 18 == header size: interrupted write?
-wpanic turns this warning into a panic: cipherSize 18 == header size: interrupted write?

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