Allow 'mvn test' to work with test-jar dependencies in a reactor

Basically the issue is demonstrated by MNG-2045, but instead of running 'mvn install', you run 'mvn test'.

Test classes of dependencies should be resolved from the reactor, just as main classes, if there's no archive available.

I'm not sure how to go about this. Specifying a dependency on something with <type>test-jar</type>,
and having that dependency declare the maven-jar-plugin with the 'test-jar' goal is insufficient.
Perhaps we can just add a standard classifier that maven is aware of, in this case 'tests'. The jar packaging
would export this as a known classifier, and tells maven how it contributes to what classpath.

Since test sources are a first class citizen, just as main sources are (they have the same phases, same
elements in the pom (but differently named)).

It seems logical to me that somehow the test classes should be made available to dependencies,
if they declare a dependency with classifier 'tests'.