Migrated tests to jest

Used jest-codemods to migrate, will have to pass everything by hand before we can merge.

test/powerset/powerset.js

@@ -1,2 +0,0 @@
-const powerset = arr => arr.reduce((a, v) => a.concat(a.map(r => [v].concat(r))), [[]]);
-module.exports = powerset;