refactor(landscape): Refactor Landscape receive-commands tests (#514)

This PR concerns the Landscape service package tests that tested the
execution of commands comming from the server. It is motivated by the
attempt at adding new tests in PR #506 regarding cloud-init integration.

These tests where sub-par for two reasons:
1. They were structured in a way that was hard to read
2. They had very little granularity: only one success and one failure
test case per command.

Adding new tests was quite hard, defeating the purpose of table testing.
With this new structure, adding test cases is much simpler, so I added
quite a few of them.

A side effect of this PR is that the executor tests can no longer be ran
without the mock. We were never running them anyways so I decided
against maitaining unused code.