As an alternate way of ensuring `srand` is called only once, and allowing the user to set the seed however they like, what do you think about this instead?

* Remove `lock_seed` (and the callsite).
* Call `srand` at the start of `RSpec::Core::CommandLine#run`.

I think that's the place we want to use the seed as it runs _after_ all the spec files have been loaded (and thus, `spec_helper.rb`, if the user is using it, has been loaded) but before any hooks or examples have run.