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I have extended the maven-nunit-plugin to support the use of systemProperties. Unfortunately, as I am working from an in-house copy I can't easily make a patch file for you.
1) Addition of new Properties class.
2) Several small changes within the NUnitEnvironment and NUnitMojo classes.
3) Upgrading org.codehaus.plexus:plexus-utils from version 1.0.4 to version 1.1 (necessary to gain some enhancements to the plexus Commandline class)
1) Somewhat different POM inheritance since I'm using an in-house version. If you take the changes above, everything should work out.
By combining this change to the maven-nunit-plugin with my change to the maven-exec-plugin (See MEXEC-4) its possible to create a cross-language integration test.
My C# based NUnit test can use the .NET Process class to execute "mvn exec:java" as part of a [TestFrameworkSetUp] annotated method in an nunit test. The exec:java goal within the C# code's POM just so happens to start up a java based server. (I actually have a little C# utility class to make this easy.) The nunit test methods can then perform integration tests requiring communication with the java server process. Upon completion of the tests a [TestFrameworkTearDown] annotated method in the nunit test can shut down the server process.