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

I plan to introduce a `base_system_gems` helper to actually install gems
from `base_system_gem_path` into system gems but that would collapse
with an existing helper.

https://github.com/rubygems/rubygems/commit/714c209e62

The mirror probing spec file was moved to our regular suite, which runs
in parallel, recently. These specs rely on starting and stopping actual
servers in localhost, but this does not play nice with parallelization,
because a spec may kill the webserver another spec has created.

This commit moves mirror probing specs to not need to start servers in
localhost and do something more similar to what the other specs do.

https://github.com/rubygems/rubygems/commit/ca9a19706f

That restores support for compact index dummy implementations that only lists
versions, without checksums or dependencies.

This format is undocumented, so we may want to get rid of it in the
future. However, some of our tests rely on it, and some implementations
did use it (gems.mutant.dev at least). And the way the code was written
suggest that support was intentional.

So for now, we should restore it.

https://github.com/rubygems/rubygems/commit/0427d8c983

https://github.com/rubygems/rubygems/commit/0a168156d5

https://github.com/rubygems/rubygems/commit/0c3a65871a

https://github.com/rubygems/rubygems/commit/d8d68cc00e