tests: Fix fusermount detection in tests.t

Used to fail with

	Unsuccessful stat on filename containing newline at tests.t line 196.
	umount: /tmp/crypt-11388: Permission denied
	not ok 44 - unmount ok, mount point removed
	#   Failed test 'unmount ok, mount point removed'
	#   at tests.t line 205

now all tests pass.
tests: Port bash tests to perl

This integrates the tests written in bash into the existing perl infrastructure.
tests: Move perl tests to tests/, make them callable from top-level Makefile

Use "make test" or "make test-verbose" to run.
Note that "make test" seems to be more common than "make tests", hence the
change.

Also, use a new clean directory directory as a working area for each run
Created using mkdtemp and deleted in cleanup()
tests: Always run unit tests
README.md: Mention EncFS unique features