[MSOURCES-124] Sources and test sources should have separate artifact types

**[Peter De Maeyer](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=peterdm)** opened **[MSOURCES-124](https://issues.apache.org/jira/browse/MSOURCES-124?redirect=false)** and commented

Imagine a project that has main and test sources, resulting in 4 artifacts:
|Artifact|Type|Default classifier|
|:---|:---|:---|
|Main JAR|"jar"| |
|Test JAR|"test-jar"|"tests"|
|Main sources|"java-source"|"sources"|
|Test sources|"java-source"|"test-sources"|

Both sources and test sources have type "java-source".
As a result, it is not possible to distinguish the sources from the test sources based on type alone.
The classifier can be used to make that distinction, but the classifier can be overridden to be anything while the type is fixed.
To fix this, I suggest to introduce a separate type "java-test-source" for test sources.
The only drawback of that is backward compatibility and impact on other plugins which might use hard-coded types.
On the other hand, attaching test sources is not often used, so I don't expect it to break a lot, and it will be better in the long run.

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**Affects:** 3.2.1

**Issue Links:**
- [MNG-8601](https://issues.apache.org/jira/browse/MNG-8601) Add type test-java-source similar to type java-source
(_**"is fixed by"**_)