This is a first pass to improve the way errors are handled and raised in
bundler's tests. The goal is to clean up tests and modernize them -
these were some obvious areas that could be cleaned up.

- Instead of raising "ZOMG" in the load error tests, it now tests for
the actual error and gem raising.
- Improve error messages where applicable.
- All errors raise a specific error class, rather than falling back to a
default and just setting a message.
- Removed arguments and `bundle_dir` option from `TheBundle` class as it wasn't
actually used so therefore we don't need to raise an error for extra
arguments.
- Removed error from `BundlerBuilder`, as it won't work if it's not
`bundler`, also it never uses `name`. The only reaon `name` is passed
in is because of metaprogramming on loading the right builder. I
think that should eventually be refactored.
- Replaced and removed `update_repo3` and `update_repo4` in favor of
just `build_repo3` and `build_repo4`. Rather than tell someone writing
tests to use a different method, automatically use the right method.

https://github.com/ruby/rubygems/commit/68c39c8451

Specs that use extension gems were failing in the new job but I noticed
that they were using very non standard `extconf.rb` files.

The hack being removed here was added just to make specs pass when run
in ruby-core but it seems the underlying issue has been fixed now, and
it's causing issues with Ruby 3.3 and Windows, so necessary so I'm
removing it and moving on.

https://github.com/rubygems/rubygems/commit/5b78275f0e

https://github.com/rubygems/rubygems/commit/bb66253f2c

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

When testing under the ruby/ruby setup, mkmf.rb needs to the `$extout`
global variable set properly.

This is because, in this particular case, the `ruby.h` header needed to
compile extensions is constructed from
`$(extout)/include($arch)/ruby/config.h` but `$extout` is not set by
default.

I tried to fix this in mkmf.rb itself but I couldn't figure it. But
setting it externally to workaround the issue fixes the specs, so I'll
start with that. Also setting it externally causes issues when running
specs upstream against Ruby 2.3 (I guess because of some difference with
Ruby 2.3 mkmf.rb implementation). So I'm avoiding doing it on Ruby 2.3 to
woraround that.

https://github.com/rubygems/rubygems/commit/d782984585

This is in preparation for deprecating source-less gemfiles.

https://github.com/rubygems/rubygems/commit/d6493fa3e2

https://github.com/rubygems/rubygems/commit/e3f60d8aec

https://github.com/rubygems/rubygems/commit/4d1a0c465a

https://github.com/rubygems/rubygems/commit/746a4b3d74