Don't run tests that run flutter concurrently

[DanTup]

This took me longer to figure out than it should have! I was seeing file locking issues on Windows at the starts of tests, even teardowns all looked good. It turned out teardowns hadn't run because we had multiple suites running 🙄

I've only added it for the flutter_sdk tests since I don't think it'll be required for the others.

[DanTup]

Another option could be to copy the test app to a random folder for each test suite... Let me know if that would be better.

[devoncarew]

We may also want to look at running all our tests sequentially, for simplicity and to reduce future issues.

[DanTup]

Sure, lmk if you want me to make that change. The other tests are pretty fast anyway, so I don't think we'd add much with -j1. If the Flutter ones start to get slow though, we might want to write them into their own folders (though in fairness, the slowest part right now is by far the downloading and preparing SDKs!).