There is no need to test that deprecated command-line options
produce an error. I trust the flags package.
Also split the example_filesystem helper functions into a
separate file.
The v0.6-plaintextnames example FS lacks the GCMIV128 feature
flag, is no longer mountable and can no longer be used for testing.
Add a new "-plaintextnames" filesystem created by gocryptfs v0.7.
There have been no format changes to "-plaintextnames" since then.
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.
