Improve DartLab Integration test speed

[ryanbrandenburg]

The actual running of the tests themselves take ~8 minutes (on a passing run, longer on most failures due to waiting for timeouts), but the total run takes ~1 hour. Surely we can trim that down to something more reasonable.

1. Test Machine Deployment takes ~30 minutes.
   1. I assume we could reduce our demanded machine count from 4 to 1 (we don't/can't parallelize anything). It might not save us much time but it should save pool resources at least.
   2. I see that the machine restarts multiple times, maybe we can coordinate with Brad to reduce that down to one restart. Even better would be if the machines could somehow pre-provision (at least when pool usage was low).
2. We currently spend ~9 minutes in the "build" step. But we are building the entire Solution instead of just the VSIX and Integration tests, so the reduced NuGet Restore times and file IO might buy us a minute or two.
3. The other idea is that if we can somehow cache some of the things we need to download throughout this somewhere closer to where they'll be needed it might be helpful. This is probably something we'd have to coordinate with the DartLab folks.