spec markdown lint tests fail with mdl >= 0.12

Describe the Bug

When using mdl >= 0.12, the spec tests in spec/unit/puppet-strings/markdown_spec.rb fail as follows:

Failures:                                                                                                                                             
                                                                                                                                                      
  1) PuppetStrings::Markdown rendering fixtures does not generate markdown lint errors from the rendered markdown                                     
     Failure/Error: expect(output).to have_no_markdown_lint_errors                                                                                    
       expected that "# Reference\n\n<!-- DO NOT EDIT: This document was generated by Puppet Strings -->"... would have no markdown lint errors but go
t 593: MD012 Multiple consecutive blank lines:                                                                                                        
     Shared Example Group: "markdown lint checker" called from ./spec/unit/puppet-strings/markdown_spec.rb:56                                         
     # ./spec/unit/puppet-strings/markdown_spec.rb:48:in `block (4 levels) in <top (required)>'                                                       
                                                                                                                                                      
  2) PuppetStrings::Markdown rendering fixtures with Puppet Plans does not generate markdown lint errors from the rendered markdown                   
     Failure/Error: expect(output).to have_no_markdown_lint_errors                                                                                    
       expected that "# Reference\n\n<!-- DO NOT EDIT: This document was generated by Puppet Strings -->"... would have no markdown lint errors but go
t 631: MD012 Multiple consecutive blank lines:                                                                                                        
     Shared Example Group: "markdown lint checker" called from ./spec/unit/puppet-strings/markdown_spec.rb:105                                        
     # ./spec/unit/puppet-strings/markdown_spec.rb:48:in `block (4 levels) in <top (required)>'                                                       
                                                                                                                                                      
  3) PuppetStrings::Markdown rendering fixtures with Puppet Data Types does not generate markdown lint errors from the rendered markdown
     Failure/Error: expect(output).to have_no_markdown_lint_errors      
       expected that "# Reference\n\n<!-- DO NOT EDIT: This document was generated by Puppet Strings -->"... would have no markdown lint errors but go
t 669: MD012 Multiple consecutive blank lines: 
     Shared Example Group: "markdown lint checker" called from ./spec/unit/puppet-strings/markdown_spec.rb:119
     # ./spec/unit/puppet-strings/markdown_spec.rb:48:in `block (4 levels) in <top (required)>'

This is due to mdl changing the default indentation from 2 spaces to 3 spaces for the rule MD007. See https://github.com/markdownlint/markdownlint/commit/67f8312aae3390df77f82c8054560bc0e8e27169

Expected Behavior

The rspec tests should pass.

Steps to Reproduce

1. Clone puppet-strings 4.2.1
2. Update the Gemfile temporarily to install mdl 0.12 or newer
3. Update installed gems in bundle
4. Use bundle to run the spec tests

Environment

• Version: 4.2.1
• OS: Debian sid

Start with the temporary mdl version change in the Gemfile and the failing examples in spec/unit/puppet-strings/markdown_spec.rb. Run the markdown rendering specs with mdl 0.12 or newer, then verify that the rendered fixtures produce no lint errors and the RSpec tests pass.