Refactoring Sandboxing to be in RSpec support

rspec-core changes for https://github.com/rspec/rspec-core/issues/1798

rspec-support changes in https://github.com/rspec/rspec-support/pull/137
As you noticed, the sandboxing logic winds up being in the stacktraces that are specified as part of the formatter integration specs.  As a result, if this line goes here, and we add or delete any lines above this one in this file, those specs will fail.  For this reason, we put the sandboxing spec setup stuff into its own `spec/support` file that is both less likely to be changed and also has this around hook at the very top of the file so that it's always on the same line regardless of what else in the file changes.