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

If you abort running test suite with a quick double Ctrl-C, tmp files
will be left around, and they will interfere with the next test run.

To avoid this, make sure to clear them once at the beginning of the test
suite.

### Before

```
$ bin/parallel_rspec
16 processes for 175 specs, ~ 11 specs per process
.............................................................................................^C^C

Finished in 19.45 seconds (files took 0.42722 seconds to load)
94 examples, 0 failures

(... turbo tests backtrace ...)

$ bin/parallel_rspec
16 processes for 175 specs, ~ 11 specs per process
.F....F....F...F......^C

Failures:

(... failures' details ...)
```

### After

```
$ bin/parallel_rspec
16 processes for 175 specs, ~ 11 specs per process
.................................................................................^C^C

Finished in 18.18 seconds (files took 0.4383 seconds to load)
82 examples, 0 failures

(... turbo tests backtrace ...)

$ bin/parallel_rspec
16 processes for 175 specs, ~ 11 specs per process
................................................................................^C^C

Finished in 8.79 seconds (files took 0.45187 seconds to load)
80 examples, 0 failures

(... turbo tests backtrace ...)
```

https://github.com/rubygems/rubygems/commit/6767a52711

https://github.com/rubygems/rubygems/commit/dafe50f171