Run template tests for kitchen sink in parallel

[julienp]

Having too many of the tests running at the same time causes disk space issues.

https://github.com/pulumi/pulumi-docker-containers/actions/runs/10099404024/job/27928516574#step:10:254

[tgummerer]

In other places we're using jlumbroso/free-disk-space@main to clean up unnecessary stuff from the action runners. Might be worth trying to use here as well.