Fix test in clojure ETL exercise

For some reason perhaps only known to Rich Hickey,

```clojure
(clojure.string/split "abc" #"") ; => ["" "a" "b" "c"]
```

(i.e. includes an empty string). I don't think this is what we want for the ETL test - and in fact the current example solution does not pass the tests for this reason. I've updated the tests to use `re-seq` instead, which gets us the letters minus the blank space, and I've simplified the example solution a little as it didn't need nested for loops.