Testing API: allow extension to control sort-order of `TestController`s it contributes

Issue Type: <b>Feature Request</b> If an extension creates more than one TestController and populates each with at least one TestItem then the controllers are sorted by `label`. It would be good to let an extension sequence them differently. A low-cost solution might be to sort controllers by `id` instead of `label`. VS Code version: Code - Insiders 1.64.0-insider (2c967bf892d52550fabae08347faae57a4539b03, 2022-01-24T05:15:04.924Z) OS version: Windows_NT x64 10.0.19044 Restricted Mode: No <!-- generated by issue reporter -->