[SUREFIRE-2218] Add support for fetching test classes from a jar

**[Sebb](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=sebb@apache.org)** opened **[SUREFIRE-2218](https://issues.apache.org/jira/browse/SUREFIRE-2218?redirect=false)** and commented

Surefire provides a way to test classes that are in a jar.

The parameter additionalClasspathElements can used to specify the jar (and the 
classesDirectory parameter can be redirected to prevent access to the default classes)
 
It would be useful to be able to do the same for the test classes, i.e. provide a parameter that can be used to update the classpath used to find tests

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No further details from [SUREFIRE-2218](https://issues.apache.org/jira/browse/SUREFIRE-2218?redirect=false)

Start by reviewing how Surefire handles the existing additionalClasspathElements and classesDirectory parameters when locating tests. Identify the parameter and classpath-resolution entry points involved, then verify that test classes can be fetched from a jar without accessing the default classes directory; the issue provides no specific files or tests to run.

Written by the indexing model from the issue text.