Declare kitchen-inspec to be run serially

[clintoncwolfe]

Description

Informs test-kitchen that the inspec verifier should not be run multithreaded. This is because Chef InSpec is not thread-safe, due to RSpec not being thread safe; when run multithreaded, random test failures or test count inaccuracies can occur, as well as random No method each for nil::NilClass errors. See #167 and https://github.com/test-kitchen/test-kitchen/issues/1530 for details.

~PR set as draft until https://github.com/test-kitchen/test-kitchen/pull/1678 merges and the TK version is bumped, because this needs to have an updated version pin against test-kitchen in the kitchen-inspec gemspec.~ TK 2.7.0 includes the feature.

Issues Resolved

Closes #167

Check List

• [ ] New functionality includes tests
• [ ] All tests pass
• [x] All commits have been signed-off for the Developer Certificate of Origin. See https://github.com/chef/chef/blob/master/CONTRIBUTING.md#developer-certification-of-origin-dco
• [ ] PR title is a worthy inclusion in the CHANGELOG

[tas50]

This either needs a if responds wrapping or the kitchen dep bumped to 2.7