Add integration tests for passing arguments to commands

[PatrickLehnerXI]

The issues found and fixed in #5 showed that passing arguments to commands isn't trivial, and especially may behave differently between OSs.

We should add some test coverage (at least in the integration tests, but maybe also unit tests?) to make sure that running commands with arguments does work, esp. the non-trivial cases (such as arguments containing spaces, containing quotes, containing backslashes).

[PatrickLehnerXI]

These test should generally try covering all public functions provided by devcmd.

For example, when adding the execToString and execInTty functions, there was a bug that the methods in ProcessExecutor weren't properly bound, which wasn't caught by the existing tests at the time. (See c24b4f8 )