Close test classloaders correctly
Close test classloaders correctly

While testing some classloader changes, I realized that we didn't always
close the test classloaders because they didn't necessarily extend
URLClassLoader, but instead implemented AutoCloseable.

Bonus: don't set the context classloader. It turns out that the test
framework does that anyway inside of trl.run so it was pointless to do
in Defaults.scala.

xuwei-k/xsbt,sbt/sbt,sbt/sbt,xuwei-k/xsbt,mdedetrich/sbt,mdedetrich/sbt,mdedetrich/sbt,sbt/sbt,sbt/sbt,xuwei-k/xsbt,mdedetrich/sbt,sbt/sbt,mdedetrich/sbt,xuwei-k/xsbt,mdedetrich/sbt,mdedetrich/sbt,sbt/sbt,xuwei-k/xsbt,sbt/sbt