Add some regression tests for some popular custom formatter gems.

For #1291.
Allow legacy formatters to access attributes from `initialize`.

This is necessary for the Fuubar formatter, among others.
Don't mistakenly add the progress formatter.

The act of adding a legacy formatter triggered a deprecation
warning which called `setup_defaults`, which added the progress
formatter because we hadn't stored the formatter in `@formatters`
yet. By moving the deprecation until after we store the formatter
in `@formatters`, it avoids this problem.
Merge pull request #1309 from rspec/formatter-regression-tests-prep-for-merge

Formatter regression tests prep for merge