[SPARK-7805] [SQL] Move SQLTestUtils.scala and ParquetTest.scala to src/test

https://issues.apache.org/jira/browse/SPARK-7805

Because `sql/hive`'s tests depend on the test jar of `sql/core`, we do not need to store `SQLTestUtils` and `ParquetTest` in `src/main`. We should only add stuff that will be needed by `sql/console` or Python tests (for Python, we need it in `src/main`, right? davies).

Author: Yin Huai <yhuai@databricks.com>

Closes #6334 from yhuai/SPARK-7805 and squashes the following commits:

af6d0c9 [Yin Huai] mima
b86746a [Yin Huai] Move SQLTestUtils.scala and ParquetTest.scala to src/test.

project/MimaExcludes.scala

@@ -133,7 +133,10 @@ object MimaExcludes {
               "org.apache.spark.sql.parquet.TestGroupWriteSupport"),
             ProblemFilters.exclude[MissingClassProblem]("org.apache.spark.sql.CachedData"),
             ProblemFilters.exclude[MissingClassProblem]("org.apache.spark.sql.CachedData$"),
-            ProblemFilters.exclude[MissingClassProblem]("org.apache.spark.sql.CacheManager")
+            ProblemFilters.exclude[MissingClassProblem]("org.apache.spark.sql.CacheManager"),
+            // TODO: Remove the following rule once ParquetTest has been moved to src/test.
+            ProblemFilters.exclude[MissingClassProblem](
+              "org.apache.spark.sql.parquet.ParquetTest")
           ) ++ Seq(
             // SPARK-7530 Added StreamingContext.getState()
             ProblemFilters.exclude[MissingMethodProblem](

sql/core/src/main/scala/org/apache/spark/sql/test/README.md

+README
+======
+
+Please do not add any class in this place unless it is used by `sql/console` or Python tests.
+If you need to create any classes or traits that will be used by tests from both `sql/core` and
+`sql/hive`, you can add them in the `src/test` of `sql/core` (tests of `sql/hive`
+depend on the test jar of `sql/core`).