I have added a subset of fsstress-gocryptfs.bash to EncFS as
fsstress-encfs.sh, improving the code a bit.
This change forward-ports these improvements to
fsstress-gocryptfs.bash.
Running these tests from integration_tests' TestMain() was awkward
because they were run twice with unchanged settings.
integration_tests tests everything with OpenSSL and with native
Go crypto, but this does not take affect for the example filesystems.
To make this work, test_helpers is also split into its own package.
