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 CI as it does not support
FUSE.

Expected output:

	$ tests/run.sh
	1 Running unit tests: OK
	*** running test_corruption.sh
	2 Reading file: OK
	3 Reading corrupted file: OK
	4 Reading file with MAC: OK
	5 Corruption with MAC returns IO error: OK
	*** running test_file_expansion.sh
	6 Creating files of different sizes: : OK
	7 Growing file: OK
	8 Internal modification: OK
	*** All tests OK
Compare MACs in constant time to prevent timing attacks

Fixes bug #12.
Merge pull request #23 from rfjakob/master

Add integration tests & Compare MACs in constant time