Extract sandboxing logic into a separate file.

The formatting specs rely on a particular expected
backtrace, which depends on which lines in particular
bits of the sandboxing code are on. Before, changes
to `spec_helper` would require updating the formatting
support expected backtraces. This change will isolate it.
Address ruby warnings from spec files.

For rspec/rspec-support#106.
Make the appropriate specs pending.

When running against rspec/rspec-support#106,
the warnings are only triggered for particular
cases that we can’t easily solve, so only pend
those specific specs.
Merge pull request #1683 from rspec/fix-spec-warnings

Fix spec warnings