Avoid rspec around hooks. Prefer before/after hooks.

RSpec's around hooks are weird and are not done correctly like ActiveSupport::Callbacks around hooks.
Abstract some of the FakeFS machinery.
Simplify the usage of FakeFS from the config/database spec.

Based on the abstraction from the previous commit.
Be sure Librarian::Specfile#path is a Pathname.
Fix the arguments to Librarian::Dsl#run.

To handle the case where it is passed 0 or 1 arguments.
Delegate handling the specfile all the way down the chain.

The Librarian::Dsl delegates handling the specfile to Librarian::Dsl::Receiver. Let it delegate the type-casing too.
Use #instance_exec in the DSL receiver.

Rather than manually fiddling with bindings.
Librarian::Specfile should pass its path to the DSL.
Move the spec support constants under the Support namespace.

For good housekeeping.