Stop build before moving on to the next test

[basil]

Not sure if this is related to failures like #119, but I noticed this in the logs:

  42.990 [id=2180]	INFO	hudson.model.Run#execute: testConcurrentBuilds #1 aborted
java.lang.InterruptedException: sleep interrupted
	at java.lang.Thread.sleep(Native Method)
	at hudson.plugins.emailext.ExtendedEmailPublisherTest$SleepOnceBuilder.perform(ExtendedEmailPublisherTest.java:1317)
	at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
	at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:741)
	at hudson.model.Build$BuildExecution.build(Build.java:206)
	at hudson.model.Build$BuildExecution.doRun(Build.java:163)
	at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:504)
	at hudson.model.Run.execute(Run.java:1894)
	at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
	at hudson.model.ResourceController.execute(ResourceController.java:97)
	at hudson.model.Executor.run(Executor.java:428)

At the very least it is unhygienic for one test to leave a build running as we move on to subsequent tests. We should stop the build before moving on to the next test.