Concurrency tests

Consider adding `bank-account` and `parallel-letter-frequency` tests with concurrency support.

On unix platforms we will need `cthreads`

https://wiki.freepascal.org/Multithreaded_Application_Tutorial#Units_needed_for_a_multi-threaded_application

https://www.freepascal.org/docs-html/rtl/cthreads/index.html

```
{$ifdef unix}
  cthreads,
{$endif}
```
