[MINOR][SQL] Fix substr/substring testcases.

## What changes were proposed in this pull request?

This PR fixes the following two testcases in order to test the correct usages.
```
checkSqlGeneration("SELECT substr('This is a test', 'is')")
checkSqlGeneration("SELECT substring('This is a test', 'is')")
```

Actually, the testcases works but tests on exceptional cases.
```
scala> sql("SELECT substr('This is a test', 'is')")
res0: org.apache.spark.sql.DataFrame = [substring(This is a test, CAST(is AS INT), 2147483647): string]

scala> sql("SELECT substr('This is a test', 'is')").collect()
res1: Array[org.apache.spark.sql.Row] = Array([null])
```

## How was this patch tested?

Pass the modified unit tests.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #11963 from dongjoon-hyun/fix_substr_testcase.

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

@@ -213,8 +213,8 @@ class ExpressionToSQLSuite extends SQLBuilderTest with SQLTestUtils {
     checkSqlGeneration("SELECT space(2)")
     checkSqlGeneration("SELECT split('aa2bb3cc', '[1-9]+')")
     checkSqlGeneration("SELECT space(2)")
-    checkSqlGeneration("SELECT substr('This is a test', 'is')")
-    checkSqlGeneration("SELECT substring('This is a test', 'is')")
+    checkSqlGeneration("SELECT substr('This is a test', 1)")
+    checkSqlGeneration("SELECT substring('This is a test', 1)")
     checkSqlGeneration("SELECT substring_index('www.apache.org','.',1)")
     checkSqlGeneration("SELECT translate('translate', 'rnlt', '123')")
     checkSqlGeneration("SELECT trim('  SparkSql ')")