Exessive run time on simple test case

I was writing some tests when they unexpectedly started taking an inordinate amount of time to run. Up to from 2 seconds to about 15 minutes. I did some digging and traced it down to some string truncation. Here's a test which replicates the issue:

```elm
n : Int
n = 64

suite : Test
suite =
describe "Example suite"
[ fuzz string "Truncation test" <|
\s -> Expect.equal s (String.left n s)
]
```

The amount of time appears to grow drastically as `n` is increased. 64 takes a number of seconds on my machine and I'm too impatient to see how long 128 takes, but it's a while. I suspect the string shrinker is responsible but I'm not sure.