Moves data_tests schema definition out of config definition in seeds

[franloza]

Issue

Fixes #220

Description

This PR moves the data_tests schema definition from config definition to root definition in seeds schema definition.
This is aligned with the dbt documentation for Seed properties

seeds:
  - name: <string>
    description: <markdown_string>
    config:
      <seed_config>: <config_value>
      docs:
        show: true | false
        node_color: <color_id> # Use name (such as node_color: purple) or hex code with quotes (such as node_color: "#cd7f32")
    data_tests:
      - <test>
      - ... # declare additional tests
    columns:
      - name: <column name>
        description: <markdown_string>
        quote: true | false
        data_tests:
          - <test>
          - ... # declare additional tests
        config:
          meta: {<dictionary>}
          tags: [<string>]

      - name: ... # declare properties of additional columns

  - name: ... # declare properties of additional seeds

data_tests is not a valid configuration for Seed configurations.