[MJAVADOC-647] Support for Split jdk 7 during build

**[Jeremy Landis](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=hazendaz)** opened **[MJAVADOC-647](https://issues.apache.org/jira/browse/MJAVADOC-647?redirect=false)** and commented

Currently it is possible to split the source / test code to compile differently.  Example use case is requirement to cross compile source target to jdk 7 and test with jdk 8.  This works fine, but the javadoc plugin doesn't seem to handle this very well. The moment java 8 items such as lambdas are introduced, it fails on javadoc generation on test side only.  I'm not sure how the pre java 9 setup is, but the post java 9 setup seems to rely only on maven.compiler.release and it should also take into account maven.compiler.testRelease.  Is it possible to have this added?

I am working around this by forcing the higher javadoc solution directly on the plugin configuration.  My preference is to not have to make special overrides when the simple flags suffice.

So for java 9+, asking that this handles

${java.release.version}
${java.test.release.version}

properly.

I suspect although didn't see in the source, that for java 8 or earlier it is either using something on its own or somehow relying on the old source/target settings but doesn't use the test variation during test java doc creation.

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**Affects:** 3.2.0

No source file or test is named. Start by tracing the javadoc plugin's Java 9+ handling of maven.compiler.release and test-side javadoc generation, then check the Java 8-and-earlier path. Done means the plugin respects maven.compiler.testRelease alongside maven.compiler.release without requiring a plugin override.

Written by the indexing model from the issue text.