tests: add TestPasswdMasterkeyStdin
tests: add TestPasswdMasterkeyStdin

Tests that `gocryptfs -passwd -masterkey=stdin` works.
This was fixed by ff04b1d83ab1201.

Fixes https://github.com/rfjakob/gocryptfs/issues/461

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