Add Tests for Definitions

[srfc]

Issue

Add tests for all definition classes, objects, traits.

• [x] ColumnDef[T] and UntypedColumnDef
  • ensure equals() (and similar) and hashCode() work correctly
  • ensure clone() works correctly
  • demonstrate ClassTag[T] usage
• [ ] ~ColumnRelation~ (deprecated with #24)
• [x] RowRelation
• [x] Record

Problem Description

We don't have tests yet.

Supporting Information

n/a

[CodeLionX]

hint for ColumnDef[T]#equals() test: asserts in TestApplication

[CodeLionX]

I guess this was fixed with #31 and #32. All following PRs should update corresponding tests!