Remove deprecated `::File.exists?` Using `::File.exist?` instead.

Also fixed bug with symlinks what was checked the using `exists?` and `exist?` at same time, but in specs was mocked only `exists?`
This code required to fix current spec(`spec/unit/provider/link_spec.rb`).
The problem was in the using `::File.exist?`, but was mocked only `::File.exists?` methods. So after I removed mock for `exists?` and made for `exist?` specs was broken. 
```ruby
describe "when the target is a regular old file" do
# .... code ....
```