[SPARK-13023][PROJECT INFRA][FOLLOWUP][BRANCH-1.6] Unable to check root module ending up failure of Python tests

## What changes were proposed in this pull request?

This PR fixes incorrect checking for `root` module (meaning all tests).

I realised that #13806 is being failed due to this one.

The PR corrects two files in `sql` and `core`. Since it seems fixing `core` module triggers all tests by `root` value from `determine_modules_for_files`.
So, `changed_modules` becomes as below:

```
['root', 'sql']
```

and `module.dependent_modules` becaomes as below:

```
['pyspark-mllib', 'pyspark-ml', 'hive-thriftserver', 'sparkr', 'mllib', 'examples', 'pyspark-sql']
```

Now, `modules_to_test` does not include `root` and this checking is skipped but then both `changed_modules` and `modules_to_test` are being merged after that. So, this includes `root` module to test.

This ends up with failing with the message below (e.g. https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/60990/consoleFull):

```
Error: unrecognized module 'root'. Supported modules: pyspark-core, pyspark-sql, pyspark-streaming, pyspark-ml, pyspark-mllib
```

## How was this patch tested?

N/A

Author: hyukjinkwon <[email protected]>

Closes #13845 from HyukjinKwon/fix-build-1.6.

Author: HyukjinKwon

dev/run-tests.py

@@ -113,10 +113,11 @@ def determine_modules_to_test(changed_modules):
     modules_to_test = set()
     for module in changed_modules:
         modules_to_test = modules_to_test.union(determine_modules_to_test(module.dependent_modules))
+    modules_to_test = modules_to_test.union(set(changed_modules))
     # If we need to run all of the tests, then we should short-circuit and return 'root'
     if modules.root in modules_to_test:
         return [modules.root]
-    return modules_to_test.union(set(changed_modules))
+    return modules_to_test
 
 
 def determine_tags_to_exclude(changed_modules):