Run same test in parallel by multiple test executors

[plajjan]

While working on a new module (that uses a C module under the hood) with some test I realized that I'm probably doing something that's not thread safe. I can run any single test for an extended period of time and perf tests runs fine (it runs one test at a time) but as soon as I run the normal acton test I'm getting failures, which indicates that there is some interaction between them. I reckon it's not thread safe enough.

Now I want our default execution strategy to be such that we exercise a function multiple times to get better timing measurements but also find flaky tests. I think we should extend this to also run the same test function in parallel so that we increase the chance of finding thread issues. It would only be applicable for things with C function (well, presuming no bugs in RTS / builtins / stdlib) but still, that's something, right?