Adds initial ruleset package migrated from common/v2/ruleset.

There is no functional change aside from renaming `test_ruleset` to
`testdata` to conform to standard conventions and splitting out package
to allow separate versioning and ownership from `common`

See original https://gitlab.com/gitlab-org/security-products/analyzers/common/-/tree/f4a306faf21ab079075533d9be3353c7b02978cb/ruleset

Relates to gitlab-org/gitlab#211819