Custom matchers for testing

[jserranohidalgo]

Testing might be improved with custom matchers.

For instance, instead of:

  val df = List((User("John", "doe", 34), 1))
      .toDF("col", "delete")
      .select("col")
  ...
   it("throws an error if the sub column is not of the provided type") {
      colStruct("col")
        .getChild[String]("age")
        .elem
        .run(df)
        .toEither
        .left
        .value
        .head shouldBe ColumnTypeError("col.age", StringType, IntegerType)

it'd be nice to write something like this:

   it("..."){
      df.select(colStruct("col").getChild[String]("age")) should 
          failWith(ColumnTypeError("col.age", StringType, IntegerType))
   }

In general, testing should not expose implementation details, and stick itself to the programmer API whenever possible.