### What changes were proposed in this pull request?
This PR proposes to move the doctests in `registerJavaUDAF` and `registerJavaFunction` to the proper unittests that run conditionally when the test classes are present.
Both tests are dependent on the test classes in JVM side, `test.org.apache.spark.sql.JavaStringLength` and `test.org.apache.spark.sql.MyDoubleAvg`. So if you run the tests against the plain `sbt package`, it fails as below:
```
**********************************************************************
File "/.../spark/python/pyspark/sql/udf.py", line 366, in pyspark.sql.udf.UDFRegistration.registerJavaFunction
Failed example:
spark.udf.registerJavaFunction(
"javaStringLength", "test.org.apache.spark.sql.JavaStringLength", IntegerType())
Exception raised:
Traceback (most recent call last):
...
test.org.apache.spark.sql.JavaStringLength, please make sure it is on the classpath;
...
6 of 7 in pyspark.sql.udf.UDFRegistration.registerJavaFunction
2 of 4 in pyspark.sql.udf.UDFRegistration.registerJavaUDAF
***Test Failed*** 8 failures.
```
### Why are the changes needed?
In order to support to run the tests against the plain SBT build. See also https://spark.apache.org/developer-tools.html
### Does this PR introduce _any_ user-facing change?
No, it's test-only.
### How was this patch tested?
Manually tested as below:
```bash
./build/sbt -DskipTests -Phive-thriftserver clean package
cd python
./run-tests --python-executable=python3 --testname="pyspark.sql.udf UserDefinedFunction"
./run-tests --python-executable=python3 --testname="pyspark.sql.tests.test_udf UDFTests"
```
```bash
./build/sbt -DskipTests -Phive-thriftserver clean test:package
cd python
./run-tests --python-executable=python3 --testname="pyspark.sql.udf UserDefinedFunction"
./run-tests --python-executable=python3 --testname="pyspark.sql.tests.test_udf UDFTests"
```
Closes#28795 from HyukjinKwon/SPARK-31965.
Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
56264fb5d3