Cleanup after tests to avoid OOME: Metaspace in fast-run testOnly sequence

After these changes, this project no longer exhibits the problems
discussed in sbt/sbt#2056

Author: retronym

build.sbt

@@ -29,7 +29,11 @@ def commonSettings: Seq[Setting[_]] = Seq(
   },
   publishArtifact in Compile := true,
   publishArtifact in Test := false,
-  parallelExecution in Test := false
+  parallelExecution in Test := false,
+  testOptions in Test += {
+    val log = streams.value.log
+    Tests.Cleanup { loader => cleanupTests(loader, log) }
+  }
 )
 
 val mimaSettings = Def settings (
@@ -232,3 +236,21 @@ def customCommands: Seq[Setting[_]] = Seq(
       state
   }
 )
+
+// TODO move into sbt-house-rules?
+def cleanupTests(loader: ClassLoader, log: sbt.internal.util.ManagedLogger): Unit = {
+  // shutdown Log4J to avoid classloader leaks
+  try {
+    val logManager = Class.forName("org.apache.logging.log4j.LogManager")
+    logManager.getMethod("shutdown").invoke(null)
+  } catch {
+    case _: Throwable =>
+      log.warn("Could not shut down Log4J")
+  }
+  // Scala Test loads property bundles, let's eagerly clear then from the internal cache
+  // TODO move into SBT itself?
+  java.util.ResourceBundle.clearCache(loader)
+  // Scala Test also starts TimerThreads that it doesn't eagerly cancel. This can weakly retain
+  // metaspace until a full GC.
+  System.gc()
+}