Exportable schema repository tests
Created by: ottomata
This PR imports and slightly refactors schema repository tests developed in https://gerrit.wikimedia.org/r/c/mediawiki/event-schemas/+/534702. These tests are now exported by @wikimedia/jsonschema-tools as a 'tests' module. A user schema repository can run jsonschema-tools based tests against it's own configured schemaBasePath.
See README.md for more info.