[MINOR] [SQL] Re-enable `explode()` and `json_tuple()` testcases in ExpressionToSQLSuite

## What changes were proposed in this pull request?

Since [SPARK-12719: SQL Generation supports for generators](https://issues.apache.org/jira/browse/SPARK-12719) was resolved, this PR enables the related testcases: `explode()` and `json_tuple()`.

## How was this patch tested?

Pass the Jenkins tests (with re-enabled test cases).

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12329 from dongjoon-hyun/minor_enable_testcases.

sql/hive/src/test/scala/org/apache/spark/sql/hive/ExpressionToSQLSuite.scala

@@ -92,8 +92,7 @@ class ExpressionToSQLSuite extends SQLBuilderTest with SQLTestUtils {
     checkSqlGeneration("SELECT abs(15), abs(-15)")
     checkSqlGeneration("SELECT array(1,2,3)")
     checkSqlGeneration("SELECT coalesce(null, 1, 2)")
-    // wait for resolution of JIRA SPARK-12719 SQL Generation for Generators
-    // checkSqlGeneration("SELECT explode(array(1,2,3))")
+    checkSqlGeneration("SELECT explode(array(1,2,3))")
     checkSqlGeneration("SELECT greatest(1,null,3)")
     checkSqlGeneration("SELECT if(1==2, 'yes', 'no')")
     checkSqlGeneration("SELECT isnan(15), isnan('invalid')")
@@ -200,8 +199,7 @@ class ExpressionToSQLSuite extends SQLBuilderTest with SQLTestUtils {
     checkSqlGeneration("SELECT locate('is', 'This is a test', 3)")
     checkSqlGeneration("SELECT lpad('SparkSql', 16, 'Learning')")
     checkSqlGeneration("SELECT ltrim('  SparkSql ')")
-    // wait for resolution of JIRA SPARK-12719 SQL Generation for Generators
-    // checkSqlGeneration("SELECT json_tuple('{\"f1\": \"value1\", \"f2\": \"value2\"}','f1')")
+    checkSqlGeneration("SELECT json_tuple('{\"f1\": \"value1\", \"f2\": \"value2\"}','f1')")
     checkSqlGeneration("SELECT printf('aa%d%s', 123, 'cc')")
     checkSqlGeneration("SELECT regexp_extract('100-200', '(\\d+)-(\\d+)', 1)")
     checkSqlGeneration("SELECT regexp_replace('100-200', '(\\d+)', 'num')")