csl: add stack traces to asserts in sbt

Summary: Problem / Solution

In cases where we use asserts in java tests, it's useful
to see the stack traces when they fail so we can more
easily debug the offending test.

Differential Revision: https://phabricator.twitter.biz/D87482

build.sbt

@@ -53,6 +53,9 @@ val sharedSettings = Seq(
   // This is bad news for things like com.twitter.util.Time
   parallelExecution in Test := false,
 
+  // -a: print stack traces for failing asserts
+  testOptions += Tests.Argument(TestFrameworks.JUnit, "-a"),
+
   // Sonatype publishing
   publishArtifact in Test := false,
   pomIncludeRepository := { _ => false },