Start parallelizing most of the acceptance tests
The only ones which are not yet resource.ParallelTests are the deploy key tests
which reuse keys and some instance-based resources / data sources.
It brings down the CI testing time a couple of minutes, but not too drastically. This is mainly because the Ci machines "only" provide 2 CPU cores which is then used by go test. Manually increasing it for Ci led to a lot of issues somehow and I wasn't able to debug it yet.
The better news is that on my local machine testing time is down to ~6 minutes, which more than half in my case.
My suggestion would be to merge this for now and improve on follow ups.
Another problem is that because we perform setups outside of the resource.ParallelTest it's not parallelized and takes up a lot of time.
Therefore, I've created https://github.com/hashicorp/terraform-plugin-sdk/issues/979 - maybe you can also chime in there if you have any good ideas :)