Tests for Chronic::Parser#pre_normalize

I added some tests to the beta branch to understand how the pre_normalize method works. They're quick and dirty tests, but they might be useful.

A couple of things stood out.

```ruby
Chronic::Parser.new.pre_normalize('a quarter to six')
#=> '1 / 4 minutes past 6'

Chronic::Parser.new.pre_normalize('three quarters until the report comes in')
#=> '3 q until the report comes in'

Chronic::Parser.new.pre_normalize('three quarters till the report comes in')
#=> '3 / 4 minutes past the end of the year'
```