Initial user-like testing (before release 0.8.0)

[foresterre]

Make a test suite which tests user like cli options and try to verify whether results are correct.

(1) Use Command or similar to run stuff like cargo run -- <options or flags, for example cargo run -- -H index.

Or:

(2) Make the Clap App return from a function or similar and put the other items in main() also in a function (say A()). Then you can call the clap app with get_matches_from(...) and call A() without having to call main.

• (+) Can be used as a normal test
• (+) Does not rely on Command / OS semantics
• (-) Is not fully run like a user since it doesn't call the sic executable

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Choice:
As process::exit()/1 is used for some commands (e.g. after printing the license or a help command), I used (1) for the commands which do this; (2) would not have returned a value and instead quits out of testing for all tests in the particular suite.
A better solution than using process::exit()/1 shouldbe used in the future.
See tracking issue #? for this.

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This issue tracks an initial version of the above; just enough to have some idea of whether 0.8.0 can be released.

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PR: #73, #74, #75
Succeeding tracking issue: #77

[foresterre]

Initial work: #73, makes it possible to run the application with custom arguments like described in option (2).

[foresterre]

As #75 was merged to master; future updates and plans will happen in #77.