...and add tests for checking that gocryptfs.diriv
does not get created.
The main "integration_tests" package has become quite
big and convoluted over time.
This small separate package should make writing tests
for "-plaintextnames" easier.
As seen in "fusefrontend: fix PlaintextNames versions of Mkdir, Rmdir",
we need more of them.
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.
