StartFunctionsHostAndWaitForError isn't waiting for the error correctly

[Charles-Gagnon]

https://github.com/Azure/azure-functions-sql-extension/blob/triggerbindings/test/Integration/SqlTriggerBindingIntegrationTests.cs#L251

We create a TaskCompletionSource and complete it, but we're never waiting on the Task.

This gets a tiny bit more complicated for multiple hosts if that's a consideration, but I don't feel like we need to test the error scenarios with multiple hosts currently since the functionality being tested here is just verifying that it throws immediately on startup and shouldn't really be something that matters at all for multiple functions running at the same time.