Rename schema tests --> generic tests

[jtcohen6]

See dbt-labs/dbt-core#3234, dbt-labs/dbt-core#3880

[jtcohen6]

A functional, non-cosmetic issue: we removed the 'data' tag from singular tests, so they'll no longer appear in the DAG viz following current logic:

dbt-docs/src/app/services/graph.service.js

Line 333 in da84e95

var is_data_test = node.resource_type == 'test' && _.includes(node.tags, 'data');