x/fin: bring test coverage from 75.0% and 48.4% to 80%

```console
% check-go-test -cover 80 `find ./x/fin f -name \*.go`
find: f: No such file or directory
=== COVERAGE: github.com/skhal/lab/x/fin/internal/fin
    coverage: 75.0% of statements
    threshold: 80.0%
--- FAIL
=== COVERAGE: github.com/skhal/lab/x/fin/internal/sim
    coverage: 48.4% of statements
    threshold: 80.0%
--- FAIL
```