Benchmark require_relative.

It appears not to make a difference.
But wait...using this old benchmark it does make a difference?
Benchmark the perf difference with a load path of different sizes.
Use new rspec-support requires.
Move `set` require to file where it is used.
One line per require file.
Remove benchmark file.

This benchmark suggested that `require_relative` isn't
any faster than `require`, because it put `rspec/core/lib`
onto the load path as the first dir, which made `require`
for rspec-core files O(1) like `require_relative`.

In practice, people's load path will have many more directories
and this benchmark doesn't align with that reality.

[ci skip]
Merge pull request #1348 from rspec/benchmark-require-relative

Benchmark require relative and update to new rspec-support method