test: use ava/get-port plugin instead of randomizing ports based on process id

[erezrokah]

In the CLI tests we spawn multiple HTTP servers (directly and indirectly via CLI commands). Since ava run specs in different processes we randomize port acquisition based on process id: https://github.com/netlify/cli/blob/db86f163451e0775a003ed60ab3c0dc452ef5992/tests/integration/utils/dev-server.js#L16

to avoid collisions.

With ava version 4 we can use https://github.com/avajs/get-port to ensure ports are safely acquired across test files.

Note this is blocked by a Node.js 16.13.2 bug (see https://github.com/avajs/ava/issues/2947) as we had to disable worker threads which is required for ava/get-port to worked.