From scalatest's ParallelTestExecution doc:

ScalaTest's normal approach for running suites of tests in parallel is to run different suites in parallel, but the tests of any one suite sequentially.
[...]
To make it easier for users to write tests that run in parallel, this trait runs each test in its own instance of the class. Running each test in its own instance enables tests to use the same instance vars and mutable objects referenced from instance variables without needing to synchronize. Although ScalaTest provides functional approaches to factoring out common test code that can help avoid such issues, running each test in its own instance is an insurance policy that makes running tests in parallel easier and less error prone.

This means that for each single test of the channel.states package, we instantiate one actor system, which contains two thread pools. With default settings, that's a minimum of 2*8 threads per individual test.

That's already pretty bad, and with 65cf238 (#2270) we add a factor of 3 on top of that, which makes us go past the OS limits on github CI.

(*) It's actually so bad, that tests run actually faster without parallelization!

(**) I tried to split large test files (e.g. NormalStateSpec, ClosingStateSpec) in order to still preserve some parallelization. In theory it should have been faster and used a bit more threads than the full sequential, but I guess it depends on the run.