[CLI] Fix run-cli leak which was revealed by repeated runCLI() calls during test

[brandonpayton]

Motivation for the change, related issues

Prior to this PR, the initial Playground CLI worker was not being cleaned up when Playground CLI was disposed.

Implementation details

This PR:

• Adds the initial worker and its Playground handle to the list of items to be disposed when Playground CLI is disposed.
• Prevents multiple workers from being requested when not running the server command.

Testing Instructions (or ideally a Blueprint)

• CI

[brandonpayton]

It looks like there are a couple of test timeouts and then a WP URL assertion failure due to treating a WP RC as the "latest" version. Fix here:

• [CLI] Fix null and "latest" WP version resolution and improve unzip error message #2889

[brandonpayton]

It looks like there are a couple of test timeouts and then a WP URL assertion failure due to treating a WP RC as the "latest" version. Fix here:

Actually, there is another real failure due to a broken check for the server command when --experimental-multi-worker is specified. Fixing...

[adamziel]

There's a timeout failure so I'll err on the side of cautious and won't merge yet.

[brandonpayton]

There's a timeout failure so I'll err on the side of cautious and won't merge yet.

Good instinct, @adamziel. :) There is indeed a problem with cleanup with the server command. I'll fix it.

[brandonpayton]

Also, the test-built-npm-packages test appears to be running indefinitely. Maybe it is related. Will see if there is an obvious fix there too.

[brandonpayton]

The test failures were due to trying to double-dispose of the initial worker and Playground. That is fixed now, and the tests are passing now. Let's merge.