Migrated tests to jest

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

test4/join/join.js

@@ -0,0 +1,11 @@
+const join = (arr, separator = ',', end = separator) =>
+arr.reduce(
+(acc, val, i) =>
+i === arr.length - 2
+? acc + val + end
+: i === arr.length - 1
+? acc + val
+: acc + val + separator,
+''
+);
+module.exports = join;