Fix variable inspection specs

[RobinDaugherty]

• Rename empty_binding variable to exception_binding, since it's obviously not empty.
• Fix the specs covering variable inspection. These specs were looking for the names of variables but actually, since RSpec includes in the scope of the example a copy of the example itself, it was just finding the source code of the example dumped in the HTML. I changed it to match the HTML itself rather than just the variable name, and splitting the HTML string keeps it from matching against the source of the example.
• Now that the variable inspection specs are working correctly, they would fail if run without binding_of caller. So I moved them into the conditional block so that they only run with boc.