Pick from https://github.com/rubygems/rubygems/commit/ba3adad4d80038ffd7bea015da2f11d3e8a2ff82

- this is preparation for onboarding Rust based extension gem generator

https://github.com/rubygems/rubygems/commit/d32801bdbc

  from https://github.com/rubygems/rubygems/commit/bfb0ae69776069155d2092702bfbb5a12617d85a

* `foo` => `TestFoo`
* `foo_bar` => `TestFooBar`
* `foo-bar` => `Foo::TestBar`

https://github.com/rubygems/rubygems/commit/353cdd61c3

foo     => test/test_foo.rb
foo-bar => test/foo/test_bar.rb
foo_bar => test/test_foo_bar.rb

https://github.com/rubygems/rubygems/commit/c795e5d40d

...with dashed gem name

In "bundle gem" command with dashed name gem (e.g. foo-bar) generates
`test/test_foo/bar.rb`, but this file contains undefined class `TestFoo`
and moreover, does not include in "bundle exec rake test" target.

Therefore, intentially the first test after gem created is fail, but in
case of gem name contains dash character is not.

The change doings...
(when "bundle gem foo-bar" called)

* create `test/test_foo_bar.rb`
* define `TestFooBar` class in `test/test_foo_bar.rb`

https://github.com/rubygems/rubygems/commit/5d9a69fc0f

In 2021, Ruby 2.5 and older are EOL.
We can set the default required Ruby version to 2.6.0 to
encourage people to use newer Ruby.
If the command is executed with old Ruby, it falls back to 2.3.0.
It's still possible to create a gem for older Ruby just by changing
two lines of code (one in gemspec and another is in rubocop.yml).