This commit adds a config for yamllint, mass-reformats all of
the existing Yaml testcases to pass linting, and adds a Yaml
linting job to the pull-request Github Actions workflow. A few
careful exceptions and ignores were added to the linter's
config to allow keeping our existing Yaml files with minimal
reformatting.
Signed-off-by: Philip Conrad <philipaconrad@gmail.com>