[MRESOLVER-12] Addition of unit tests for DependencySelector implementations

**[Christian Schulte](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=schulte77)** opened **[MRESOLVER-12](https://issues.apache.org/jira/browse/MRESOLVER-12?redirect=false)** and commented

The `DefaultDependencyCollector` heavily relies on various components provided by an application. There should be unit tests for testing that at least the `ScopeDependencySelector` and `OptionalDependencySelector` selector are working as stated in the Javadoc.

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**Issue Links:**
- [MRESOLVER-8](https://issues.apache.org/jira/browse/MRESOLVER-8) ScopeDependencySelector incorrectly de-selects direct dependencies
(_**"depends upon"**_)
- [MRESOLVER-9](https://issues.apache.org/jira/browse/MRESOLVER-9) DefaultDependencyCollector does not correctly handle dependency management
(_**"depends upon"**_)

Start with the DefaultDependencyCollector and the ScopeDependencySelector and OptionalDependencySelector implementations mentioned in the issue, then read their Javadocs to identify the expected behavior. Add unit tests covering those documented cases and confirm the selectors behave as stated.

Written by the indexing model from the issue text.