Refactor JS test data generating fns to use .filter() when sampling

[echeran]

This issue could become obsolete if/when we generate data straight from CLDR. But for now, with the JS code, we can:

1. Refactoring test data sampling code to a common function rather than reimplement that logic in each component's test data generator
2. Instead of manual logic for taking every nth element of a collection, reuse JS's FP functions and a one-liner (alternative formulation).

[sven-oly]

Working on this now.