FilterScorer to override more super-class methods [LUCENE-7683]

[Scorer.java](https://github.com/apache/lucene-solr/blob/master/lucene/core/src/java/org/apache/lucene/search/Scorer.java) has non-abstract non-final non-private non-static methods (getChildren, getWeight) which the [FilterScorer.java](https://github.com/apache/lucene-solr/blob/master/lucene/core/src/java/org/apache/lucene/search/FilterScorer.java) class does not override.

Proposed changes:
- Option 1: Add the missing methods.
- Option 2: Make the missing methods `final` in the non-Filter base class.
- Either way, add `TestFilterScorer.java` class similar to [TestFilterWeight.java](https://github.com/apache/lucene-solr/blob/master/lucene/core/src/test/org/apache/lucene/search/TestFilterWeight.java) class.

Optional bonus (as a separate patch?):
- TestFilterWeight, TestFilterCodecReader, TestMergePolicyWrapper and possibly other tests all have `implTestDeclaredMethodsOverridden(superClass, subClass, excusedMethods)` logic and some sort of [lucene/test-framework util](https://github.com/apache/lucene-solr/tree/master/lucene/test-framework/src/java/org/apache/lucene/util) FilterTestUtils.java class with a static implTestDeclaredMethodsOverridden method could perhaps be factored out.

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Migrated from [LUCENE-7683](https://issues.apache.org/jira/browse/LUCENE-7683) by Christine Poerschke (@cpoerschke)
Linked issues:
- #8727

Compare the non-abstract methods in lucene/core/src/java/org/apache/lucene/search/Scorer.java with lucene/core/src/java/org/apache/lucene/search/FilterScorer.java, then review lucene/core/src/test/org/apache/lucene/search/TestFilterWeight.java. Decide between adding overrides or making the base methods final, and add TestFilterScorer.java to verify the chosen behavior.

Written by the indexing model from the issue text.