tests: Add integration tests

Add a simple cli-based test infrastructure with a few tests
to prepare future code changes.
Note that these cannot be run on Travis as it does not support
FUSE.

Expected output:

	$ tests/run.sh
	Running unit tests: OK
	*** running test_corruption.sh
	Reading file: OK
	Reading corrupted file: OK
	Reading file with MAC: OK
	Testing corrupted file with MAC: OK
	*** running test_file_expansion.sh
	Creating files of different sizes: OK
	Growing file: OK
	Internal modification: OK
	*** All tests OK
Compare MACs in constant time to prevent timing attacks

Fixes bug #12.