rework spec to harness output correctly
allow override of runner output
Swap output stream to current sandbox (Read why).

Specs that use the runner need to use the current sandbox output stream
(rather than attempting to create a new one) so that inadvertant use of
the config hasn't preloaded any formatters (e.g. deprecation warnings
will cause stuff to be initialised which would then not be captured in
our specs correctly).
revert overwriting output and fixup spec to discourage it
Merge pull request #1073 from rspec/fix_formatter_output_issue

Running rspec-core's specs with `--order default` produces an error