Test External Terminal support for runInTerminal request

[weinand]

Test for #11581:

• Windows - @stevencl
• OS X - @chrmarti
• Linux - @octref

Complexity 2

In this release the implementation of the 'run in external terminal' feature ("console": "externalTerminal") of node-debug has been moved to VS Code (and is now available for all debug adapters.)
A benevolent side effect of this move is that it is now possible to configure the terminal program to use via the terminal.external.windowsExec, terminal.external.osxExec, terminal.external.linuxExec settings.

Make sure that:

• you can launch a debug target in an external terminal,
• cwd, env vars and arguments are passed correctly,
• you can configure a different terminal and things still work.

[octref]

@weinand On linux not working for other terminals like rxvt-unicode and terminator: #12791