[dartdevc] Add new bot configuration for nnbd strong
[dartdevc] Add new bot configuration for nnbd strong

At this time I have a single test in another cl designed to run with the nnbd
fork of the SDK in strong checking mode so I'm creating this single
configuration for that purpose. As I understand, all other tests that do not
require nnbd-strong will be skipped on this configuration.

I can expand this into more configurations when we have more tests coming online.

I don't think it is important to run these on all OS/browsers at the moment.

Issue: #38665
Change-Id: I284691022c1505d8a6e388fbc1b75f0074cc7164
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/121652
Commit-Queue: Nicholas Shahan <03847abd2d8aadfe859ecb98095a31f9f515dc47@google.com>
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>

dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk