string/concatenation.rb tests are misleading

Hi, the tests in `string/concatenation.rb` are quite misleading.

The fast method consists of this

```
def fast
'foo' 'bar'
end
```

That's not concatenating during calling of `fast` but on parsing the code. If you write another method just returning `foobar`, it is as fast as this method.

So I think this is not fair comparison and what you usually want is to concatenate two variables during runtime.

For this use case `concat` and `<<` are calling the same code, so they have the same performance and both are fine if you want to change the string on the left and not just get two strings concatenated. If you want a new string you can use `+`.

Some better test could be to compare `+` and String interpolation

```
Benchmark.ips do |x|
foo = 'foo'
bar = 'bar'

x.report('String#+') do
foo + bar
end

x.report('String interpolation') do
"#{foo}#{bar}"
end

x.compare!
end
```

This still has the difference that interpolation can handle nil values, while `+` cannot.