Abstract the commands and their run to make cargo-scout more testable

[o0Ignition0o]

There's a lot of cover_skip in the codebase because commands cannot be easily tested:

    // Skipped from code coverage
    // because an external command
    // cannot be easily unit tested
    #[cfg_attr(tarpaulin, skip)]

We might want to try working on our own command abstraction, that would return a Result<OutputStream, ErrorStream> so we can provide stubs for the tests.

It would allow for more coverage, and we could thus identify and reproduce cornercases easily.