Overhaul test suite

[JSAbrahams]

Issue, or potential issue

Currently, the test suite is still somewhat manageable, but as the codebase grows, this will become more of an issue. There are still some slight inconsistencies, and some things are not as elegant as they should be, such as error reporting or test setup and cleanup.

A few identified issues:

• [x] The error messages generated by util are not descriptive because we don't do anything with the results, we immediately unwrap(). This Instead, we should actually do something when for instance a file is not found.
• [x] Function names in util are a bit long and unwieldy, we should consider changing these to make these more modular.
• [x] As we slowly move to the phase of testing outputted files based on input (see #17 and more importantly #32), we need to more systematically set up en clean up tests.
• [x] Tests should return a result (Result<(), Box<std::error::Error>>)

Potential solutions

The rust-lang-nursery provides some information about testing applications.

Additional Context

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[JSAbrahams]

For now I'd consider the test "good enough". If we identify a specific improvement then we can create an issue for that, instead of this vague issue which is basically "make tests better".