[SPARK-14350][SQL] EXPLAIN output should be in a single cell

## What changes were proposed in this pull request?

EXPLAIN output should be in a single cell.

**Before**
```
scala> sql("explain select 1").collect()
res0: Array[org.apache.spark.sql.Row] = Array([== Physical Plan ==], [WholeStageCodegen], [:  +- Project [1 AS 1#1]], [:     +- INPUT], [+- Scan OneRowRelation[]])
```

**After**
```
scala> sql("explain select 1").collect()
res1: Array[org.apache.spark.sql.Row] =
Array([== Physical Plan ==
WholeStageCodegen
:  +- Project [1 AS 1#4]
:     +- INPUT
+- Scan OneRowRelation[]])
```
Or,
```
scala> sql("explain select 1").head
res1: org.apache.spark.sql.Row =
[== Physical Plan ==
WholeStageCodegen
:  +- Project [1 AS 1#5]
:     +- INPUT
+- Scan OneRowRelation[]]
```

Please note that `Spark-shell(Scala-shell)` trims long string output. So, you may need to use `println` to get full strings.
```
scala> println(sql("explain codegen select 'a' as a group by 1").head)
[Found 2 WholeStageCodegen subtrees.
== Subtree 1 / 2 ==
WholeStageCodegen
...
/* 059 */   }
/* 060 */ }

]
```

## How was this patch tested?

Pass the Jenkins tests. (Testcases are updated.)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12137 from dongjoon-hyun/SPARK-14350.

0 files changed, 0 insertions, 0 deletions