Give example group classes a friendly const name.

Practically, this gives friendly error messages.  Before,
an undefined message called from an example errored like:

undefined local variable or method `foo' for #<RSpec::Core::ExampleGroup::Nested_1::Nested_1:0x007f87d15b8078>

…and now it is:

undefined local variable or method `foo' for #<RSpec::UserGroups::RSpecCallerFilter::TheFilteringRegex:0x007f951bd1f300>
Assign the const before eval'ing the block.