Test 'UI support for TimeTravel Debugging'

[weinand]

Tests #7652:

• any OS - @aeschli

VS Code debug now supports a 'Step Back' button for those debug adapters that provide the corresponding capability. For now this is mock-debug (from Marketplace) and node-debug (builtin).

In mock-debug the 'Step Back' capability is always enabled and actually works.
Install mock-debug, create a launch config and a markdown file readme.md with some lines of text. Start debugging and then single step a bit and then 'Step Back'.

In node-debug the 'Step Back' capability is only enabled if a node runtime is used that supports stepping back (chakra). If this is not available you can simulate it by adding a "stepBack": true to the launch config. This should make the 'Step Back' button appear when starting a debug session, but pressing the button will bring up an error because regular node.js doesn't support stepping back.

Verify that the 'Step Back' button appears and is enabled when expected and that it is accessible.