Tests for the snippets in the 'README'

This is basically how I would add tests, it's the xcode free way of having tests.

```
❯ ruby tests/xctest.rb
Tests
  PASS test_readme_enumerating (0.000s)
  PASS test_readme_inject (0.000s)
  PASS test_readme_select (0.000s)
  PASS test_readme_shorthand (0.000s)
Executed 4 tests, with 0 failures (0 unexpected) in 0.001 (0.002) seconds
```

Downsides:
- it has some ruby in it
- it has 0 xcode support

Upsides:
- it has some ruby in it
- it has 0 xcode support

The only thing that I actually find annoying is having to type `ruby tests/xctest.rb` so I'd probably add a Makefile to handle that. (But since I'd be using a `Makefile` I'd probably move the compiling stuff to the makefile as well, instead of doing it in the ruby part).

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