[SPARK-18760][SQL] Consistent format specification for FileFormats

## What changes were proposed in this pull request?
This patch fixes the format specification in explain for file sources (Parquet and Text formats are the only two that are different from the rest):

Before:
```
scala> spark.read.text("test.text").explain()
== Physical Plan ==
*FileScan text [value#15] Batched: false, Format: org.apache.spark.sql.execution.datasources.text.TextFileFormatxyz, Location: InMemoryFileIndex[file:/scratch/rxin/spark/test.text], PartitionFilters: [], PushedFilters: [], ReadSchema: struct<value:string>
```

After:
```
scala> spark.read.text("test.text").explain()
== Physical Plan ==
*FileScan text [value#15] Batched: false, Format: Text, Location: InMemoryFileIndex[file:/scratch/rxin/spark/test.text], PartitionFilters: [], PushedFilters: [], ReadSchema: struct<value:string>
```

Also closes #14680.

## How was this patch tested?
Verified in spark-shell.

Author: Reynold Xin <rxin@databricks.com>

Closes #16187 from rxin/SPARK-18760.

(cherry picked from commit 5f894d23)
Signed-off-by: Reynold Xin <rxin@databricks.com>

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetFileFormat.scala

@@ -61,7 +61,7 @@ class ParquetFileFormat
 
   override def shortName(): String = "parquet"
 
-  override def toString: String = "ParquetFormat"
+  override def toString: String = "Parquet"
 
   override def hashCode(): Int = getClass.hashCode()
 

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/text/TextFileFormat.scala

@@ -43,6 +43,8 @@ class TextFileFormat extends TextBasedFileFormat with DataSourceRegister {
 
   override def shortName(): String = "text"
 
+  override def toString: String = "Text"
+
   private def verifySchema(schema: StructType): Unit = {
     if (schema.size != 1) {
       throw new AnalysisException(

sql/core/src/test/scala/org/apache/spark/sql/streaming/FileStreamSourceSuite.scala

@@ -31,7 +31,8 @@ import org.apache.spark.sql.test.SharedSQLContext
 import org.apache.spark.sql.types._
 import org.apache.spark.util.Utils
 
-class FileStreamSourceTest extends StreamTest with SharedSQLContext with PrivateMethodTester {
+abstract class FileStreamSourceTest
+  extends StreamTest with SharedSQLContext with PrivateMethodTester {
 
   import testImplicits._
 
@@ -848,13 +849,13 @@ class FileStreamSourceSuite extends FileStreamSourceTest {
         val explainWithoutExtended = q.explainInternal(false)
         // `extended = false` only displays the physical plan.
         assert("Relation.*text".r.findAllMatchIn(explainWithoutExtended).size === 0)
-        assert("TextFileFormat".r.findAllMatchIn(explainWithoutExtended).size === 1)
+        assert(": Text".r.findAllMatchIn(explainWithoutExtended).size === 1)
 
         val explainWithExtended = q.explainInternal(true)
         // `extended = true` displays 3 logical plans (Parsed/Optimized/Optimized) and 1 physical
         // plan.
         assert("Relation.*text".r.findAllMatchIn(explainWithExtended).size === 3)
-        assert("TextFileFormat".r.findAllMatchIn(explainWithExtended).size === 1)
+        assert(": Text".r.findAllMatchIn(explainWithExtended).size === 1)
       } finally {
         q.stop()
       }