Add acceptance tests

As it's impossible to use real `sfboot` tool for acceptance tests, it might be smart enough to implement some fake `sfboot` tool (using the helper, extracted in #1). This fake `sfboot` tool should behave like the real one, but just read/save a file (files) instead.

Start by reading the helper extracted in issue #1 and compare the requested fake sfboot behavior with the real tool's read/save operations. Define acceptance tests around those file operations and confirm the fake tool can stand in for sfboot without invoking the real executable.

Written by the indexing model from the issue text.