https://github.com/ruby/rubygems/commit/3ddb740969

I organized all examples the followings:

```
Total test time: 2468.41 seconds
Total files: 168

Group A: 42 files, 617.08 seconds
Group B: 42 files, 617.05 seconds
Group C: 42 files, 617.14 seconds
Group D: 42 files, 617.14 seconds
```

https://github.com/ruby/rubygems/commit/94d41e6c7c

https://github.com/ruby/rubygems/commit/4d8b1c77f4

This is a follow up to #9053. We can avoid shelling out for generating
the gem index.

https://github.com/ruby/rubygems/commit/e40bafe9f1

```
       Fetching gem metadata from https://gem.repo4/.
       Resolving dependencies...
       Resolving dependencies...
       # $? => 0
       cannot load such file -- diff/lcs
```

https://github.com/ruby/rubygems/commit/668b300261

When we need to reset system gems during specs, there's no need to
rebuild bundler, we can copy over the original gem home.

https://github.com/rubygems/rubygems/commit/7b4f80747b