tests: Move helper funtion to common.inc

Also fixes md5fh's fd leak.
Handle ENCFS6_CONFIG set to non-existing file

This used to give the confusing error

	Found config file %s, but failed to load - exiting
Log offending filename "too small" errors
tests: explicitely verify that the reverse mount worked
Disable unsafe kernel caching for --reverse

By default, the kernel caches file metadata for one second.
This is fine for EncFS' normal mode, but for --reverse, this
means that the encrypted view will be up to one second out of
date.
This causes the reverse grow tests to fail because stale stat()
data is returned.
tests: Add --reverse file grow test

This test uncovered issues caused by unsafe kernel-side and BlockFileIO caching.
Implement uniqueIV for reverse mode WiP

Work in progress: Offsets above 16KB return garbage