Convert the spec for Bundler::UpdateChecker to use project based fixtures.

[brrygrdn]

This PR extracts from #3350

It prepares the way for bundler version detection by ensuring we can readily produce builds against lockfiles with the correct BUNDLED WITH values based on test parameters.

Methodology

In order to verify the change over, I used a helper shim to force any tests that called fixture to load gemfiles to raise, e.g.

# bundler/spec/spec_helper.rb
alias non_project_fixture fixture

def fixture(*name)
  if name.any? { |folder| %w(gemfiles gemspecs lockfiles).include? folder }
    raise "Non-Project Fixture Loaded: '#{File.join(name)}'."
  end

  non_project_fixture(*name)
end

A large number of the bundler1 fixtures already existed, so it was just a case of generating any missing files, checking their naming lined up and then finally grepping the file for _fixture_name to ensure no test cases were quietly failing over to the top-level dependency files.

[brrygrdn]

Aha, I forgot I'll need to commit a stubbed version of bundler_project_dependency_files for these tests to pass, will add that now

[brrygrdn]

This method provides a forward compatible way to load any fixtures modified in this PR, I left any existing calls to project_dependency_files alone to avoid bloating the diff.

[jurre]

Right on 🤙

I've reviewed the changes to the tests, and based on them passing trust that the fixtures are 💯

[feelepxyz]

👍

[feelepxyz]

🙌