cli: make program outputs using the cli module testable in cli/testdata

[ttytm]

Changes in regard of an issue I'm encountering with the cli module and like to fix and test cover. To make development (especially testing) more accessible, this PR adds testability of program outputs that use the cli module.

Changes per commit:

• Makes the current test in cli_test.v namely fn test_long_description also testable as executable in vlib/cli/testdata/long_description.vv / ../long_description.out.
• Moves the cli modue tests that currently reside in vlib/v/slow_tests/inout/ into cli/testdata (all the cli tests take usually less than 300ms)

Testing locally after checking out the PR and making a change to one of the test / out files to ensure the tests work as they should might help to simplify reviewing.

[spytheman]

Good work. It makes the module more self contained.

The slowdown is not too bad, even on my slow i3:

[spytheman]

The change detection also works, and produces easily readable results: