Don't run Spark tests in parallel

Spark uses a global context that is shared across threads.
If we run all the tests in parallel we risk to shutdown the
context from one thread whilst is still used by the others.

Change-Id: I7e536ae6058077334b37cab4a3dfdf224bc65a65
diff --git a/build.sbt b/build.sbt
index 8e9e4c3..14e8af9 100644
--- a/build.sbt
+++ b/build.sbt
@@ -18,4 +18,6 @@
   "org.scalatest" %% "scalatest" % "3.0.1" % "test"
 )
 
-mainClass in (Compile,run) := Some("com.gerritforge.analytics.job.Main")
\ No newline at end of file
+mainClass in (Compile,run) := Some("com.gerritforge.analytics.job.Main")
+
+parallelExecution in Test := false
\ No newline at end of file