Add support for finding load time warnings

Extraction of the supporting code from rspec/rspec-core#1654.
Sorry my concern here is the standard library requires, they could include this file into their spec suite (for example maybe they're hacking with something in here) and then break production code. I was thinking something like this:

```Ruby
if !defined?(Open3) || !RSPECS_OWN_TEST_SUITE
  puts "Warning message"
end
```