Specifically seems to be maven-hpi-plugin 2.0 that's causing this, with the JENKINS-24064 fix. Now we get an error like

java.lang.AssertionError: jenkins.war is not in the classpath. If you are running this from the core workspace, run 'mvn install' to create the war image in war/target/jenkins

whenever trying to run tests. jglick says this is due to Intellij not honoring https://github.com/jenkinsci/maven-hpi-plugin/blob/137e435d0770f78edbbd2ed54d91809b7e7be5aa/src/main/resources/META-INF/plexus/components.xml#L134, so we need to figure out how to, like, get it to honor that.