Behat tests for nested gridfield

[GuySartorelli]

#383 recently introduced a new nested gridfield component.

This is quite complex, so it should have some integration tests to ensure it works as expected going forward.

Acceptance criteria

• Update the basic set up of Gridfiled extensions to include some behat test
• Behat tests exist to cover the golden path for nested gridfield
  • Basic nested gridfield for hierarchical data, such as Group or SiteTree
  • Double nested gridfield for non-hierarchical relational data (use getNestedConfig())
  • Basic nested gridfield for non-relational data (return ArrayList)
• Create a follow-up card for additional behat coverage for other components in this module

Possible specific things to test

• Sorting main gridfield
• filtering main gridfield
• Drilling down on a specific record
• Sort and filter the nested gridfield

Exclude

• Covering the rest of the module with behat tests

PRs

• MNT Object for Nested GridField testing silverstripe/silverstripe-frameworktest#179
• MNT NestedGridField behat tests #398
• MNT Change set of ids to limit in NonRelationalData test class silverstripe/silverstripe-frameworktest#181

Follow-up card

• Gridfield extensions module isn't covered by comprehensive Behat test coverage #399

[GuySartorelli]

PRs merged