[SPARK-28392][SQL][TESTS] Add traits for UDF and PostgreSQL tests to …

…share initialization

## What changes were proposed in this pull request?

This PR adds some traits so that we can deduplicate initialization stuff for each type of test case. For instance, see [SPARK-28343](https://issues.apache.org/jira/browse/SPARK-28343).

It's a little bit overkill but I think it will make adding test cases easier and cause less confusions.

This PR adds both:

```
  private trait PgSQLTest
  private trait UDFTest
```

To indicate and share the logics related to each combination of test types.

## How was this patch tested?

Manually tested.

Closes #25155 from HyukjinKwon/SPARK-28392.

Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

sql/core/src/test/resources/sql-tests/inputs/udf/pgSQL/udf-case.sql

@@ -6,14 +6,10 @@
 -- https://github.com/postgres/postgres/blob/REL_12_BETA2/src/test/regress/sql/case.sql
 -- Test the CASE statement
 --
--- This test suite contains two Cartesian products without using explicit CROSS JOIN syntax.
--- Thus, we set spark.sql.crossJoin.enabled to true.
-
 -- This test file was converted from pgSQL/case.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.
 
-set spark.sql.crossJoin.enabled=true;
 CREATE TABLE CASE_TBL (
   i integer,
   f double
@@ -269,4 +265,3 @@ SELECT CASE
 
 DROP TABLE CASE_TBL;
 DROP TABLE CASE2_TBL;
-set spark.sql.crossJoin.enabled=false;