Move simplecov config into rails_helper

[mjankowski]

Extracted from #30284

I had previously done the extraction into the .simplecov file, which seemed harmless at the time, but introduced a subtle load order bug that I finally hit on that flatware PR.

When you require simplecov, it checks if the .simplecov file exists, and if it does, immediately loads/runs it. This is fine when you do that once, but if you have multiple places that need to do different portion of the config (the flatware PR has one place for flatware to interact with the at_fork setting of simplecov and other place for the core/regular config) it means you've lost control of when to do which things.

So, this is a few changes:

• Delete .simplecov and move its contents into rails_helper (it needs to be here instead of spec_helper for similar load order reasons ... we want it loaded after the core initial rspec setup, but before all the rails loading happens)
• Add a monkey patch for a simplecov branch merge bug (see linked PR in the file)
• Small style improvement on how the formatter is assigned

I'm curious to confirm the codecov results on this one by itself - but also this felt like a good extraction from the flatware PR to keep that one focused on just adding flatware and not on unrelated simplecov config.