[SPARK-10176] [SQL] Show partially analyzed plans when checkAnswer fails to analyze

This PR takes over https://github.com/apache/spark/pull/8389.

This PR improves `checkAnswer` to print the partially analyzed plan in addition to the user friendly error message, in order to aid debugging failing tests.

In doing so, I ran into a conflict with the various ways that we bring a SQLContext into the tests. Depending on the trait we refer to the current context as `sqlContext`, `_sqlContext`, `ctx` or `hiveContext` with access modifiers `public`, `protected` and `private` depending on the defining class.

I propose we refactor as follows:

1. All tests should only refer to a `protected sqlContext` when testing general features, and `protected hiveContext` when it is a method that only exists on a `HiveContext`.
2. All tests should only import `testImplicits._` (i.e., don't import `TestHive.implicits._`)

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #8584 from cloud-fan/cleanupTests.