* Move prefix tests to golden-based

There's a bug in the file attribute (but not the content), so the prefix
test didn't catch it. Score one for golden testing I guess.

* Ensure custom prefix is applied in file attribute

The primary purpose of this commit is to simplify usage to just:

    main :: IO
    main = hspecJUnit Spec.spec

This necessitated configuration through JUNIT_-prefixed environment
variables, implemented in the new Config.Env module.

In support of this, we also get:

- hspecJUnitWith

  hspecJUnit is to hspec as hspecJUnitWith is to hspecWith. This will be
  useful in our projects that perform further per-suite configuration.

- configAvailableFormatters support, when possible

  In hspec-2.9, we can add ourselves to the -f option for conditionally
  running with our format function. This obviates JUNIT_ENABLED, but
  we'll support that too.