[SPARK-27922][SQL][PYTHON][TESTS] Convert and port 'natural-join.sql' into UDF test base

## What changes were proposed in this pull request?

This PR adds some tests converted from `natural-join.sql` to test UDFs following the combination guide in  [SPARK-27921](https://issues.apache.org/jira/browse/SPARK-27921).

<details><summary>Diff results comparing to `natural-join.sql`</summary>
<p>

```diff
diff --git a/sql/core/src/test/resources/sql-tests/results/udf/udf-natural-join.sql.out b/sql/core/src/test/resources/sql-tests/results/udf/udf-natural-join.
sql.out
index 43f2f9a..53ef177 100644
--- a/sql/core/src/test/resources/sql-tests/results/udf/udf-natural-join.sql.out
+++ b/sql/core/src/test/resources/sql-tests/results/udf/udf-natural-join.sql.out
 -27,7 +27,7  struct<>

 -- !query 2
-SELECT * FROM nt1 natural join nt2 where k = "one"
+SELECT * FROM nt1 natural join nt2 where udf(k) = "one"
 -- !query 2 schema
 struct<k:string,v1:int,v2:int>
 -- !query 2 output
 -36,7 +36,7  one   1       5

 -- !query 3
-SELECT * FROM nt1 natural left join nt2 order by v1, v2
+SELECT * FROM nt1 natural left join nt2 where k <> udf("") order by v1, v2
 -- !query 3 schema
diff --git a/sql/core/src/test/resources/sql-tests/results/udf/udf-natural-join.sql.out b/sql/core/src/test/resources/sql-tests/results/udf/udf-natural-join.
sql.out
index 43f2f9a..53ef177 100644
--- a/sql/core/src/test/resources/sql-tests/results/udf/udf-natural-join.sql.out
+++ b/sql/core/src/test/resources/sql-tests/results/udf/udf-natural-join.sql.out
 -27,7 +27,7  struct<>

 -- !query 2
-SELECT * FROM nt1 natural join nt2 where k = "one"
+SELECT * FROM nt1 natural join nt2 where udf(k) = "one"
 -- !query 2 schema
 struct<k:string,v1:int,v2:int>
 -- !query 2 output
 -36,7 +36,7  one   1       5

 -- !query 3
-SELECT * FROM nt1 natural left join nt2 order by v1, v2
+SELECT * FROM nt1 natural left join nt2 where k <> udf("") order by v1, v2
 -- !query 3 schema
 struct<k:string,v1:int,v2:int>
```

</p>
</details>

## How was this patch tested?

Tested as guided in [SPARK-27921](https://issues.apache.org/jira/browse/SPARK-27921).

Closes apache#25088 from manuzhang/SPARK-27922.

Authored-by: manu.zhang <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>

Author: manu.zhang

sql/core/src/test/resources/sql-tests/inputs/udf/udf-natural-join.sql

@@ -0,0 +1,29 @@
+-- List of configuration the test suite is run against:
+--SET spark.sql.autoBroadcastJoinThreshold=10485760
+--SET spark.sql.autoBroadcastJoinThreshold=-1,spark.sql.join.preferSortMergeJoin=true
+--SET spark.sql.autoBroadcastJoinThreshold=-1,spark.sql.join.preferSortMergeJoin=false
+
+-- This test file was converted from natural-join.sql.
+-- Note that currently registered UDF returns a string. So there are some differences, for instance
+-- in string cast within UDF in Scala and Python.
+
+create temporary view nt1 as select * from values
+  ("one", 1),
+  ("two", 2),
+  ("three", 3)
+  as nt1(k, v1);
+
+create temporary view nt2 as select * from values
+  ("one", 1),
+  ("two", 22),
+  ("one", 5)
+  as nt2(k, v2);
+
+
+SELECT * FROM nt1 natural join nt2 where udf(k) = "one";
+
+SELECT * FROM nt1 natural left join nt2 where k <> udf("") order by v1, v2;
+
+SELECT * FROM nt1 natural right join nt2 where udf(k) <> udf("") order by v1, v2;
+
+SELECT udf(count(*)) FROM nt1 natural full outer join nt2;

sql/core/src/test/resources/sql-tests/results/udf/udf-natural-join.sql.out

@@ -0,0 +1,64 @@
+-- Automatically generated by SQLQueryTestSuite
+-- Number of queries: 6
+
+
+-- !query 0
+create temporary view nt1 as select * from values
+  ("one", 1),
+  ("two", 2),
+  ("three", 3)
+  as nt1(k, v1)
+-- !query 0 schema
+struct<>
+-- !query 0 output
+
+
+
+-- !query 1
+create temporary view nt2 as select * from values
+  ("one", 1),
+  ("two", 22),
+  ("one", 5)
+  as nt2(k, v2)
+-- !query 1 schema
+struct<>
+-- !query 1 output
+
+
+
+-- !query 2
+SELECT * FROM nt1 natural join nt2 where udf(k) = "one"
+-- !query 2 schema
+struct<k:string,v1:int,v2:int>
+-- !query 2 output
+one	1	1
+one	1	5
+
+
+-- !query 3
+SELECT * FROM nt1 natural left join nt2 where k <> udf("") order by v1, v2
+-- !query 3 schema
+struct<k:string,v1:int,v2:int>
+-- !query 3 output
+one	1	1
+one	1	5
+two	2	22
+three	3	NULL
+
+
+-- !query 4
+SELECT * FROM nt1 natural right join nt2 where udf(k) <> udf("") order by v1, v2
+-- !query 4 schema
+struct<k:string,v1:int,v2:int>
+-- !query 4 output
+one	1	1
+one	1	5
+two	2	22
+
+
+-- !query 5
+SELECT udf(count(*)) FROM nt1 natural full outer join nt2
+-- !query 5 schema
+struct<udf(count(1)):string>
+-- !query 5 output
+4