add test to make sure 

I see you've removed the original tests that were the impetus for my additional tests: 3cbd1b567c287d6b1d4b1d0422d3dbccaab2c0f8 (Testing for longer length strands)

When I was initially doing this exercise I was trying to use the `-` operator, which allowed to falsely get passing tests. 

```
[a,a,b,b,b]-[a,a,b]
=> []
```

rather than:
```
=> [b,b]
```


More contextually relevant example:

```
puts "Should be 1 => #{('AGA'.chars - 'AGG'.chars).length}"
puts "Should be 1 => #{('GGACG'.chars - 'GGTCG'.chars).length}"
```

The first will result in 0, rather than 1 and the second `puts` will result in 1.

So I added two tests to make sure that we're not checking for instances of y, from x (rather than just subtracting from a continuous snippet).