Run the matrix tests from the jars

The step passed -Dmaven.test.classpath.useTestJar=true, which is not a
surefire parameter, so surefire ignored it, found no test classes and
reported "Tests run: 0, BUILD SUCCESS". Put the main and test jars on the
test classpath instead, and fail the job if no tests run.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>