Fork the JUnit tests, fixes aeron-io#96

Author: Michael Barker

build.xml

@@ -119,7 +119,7 @@
  <compilerarg value="-Xlint:unchecked"/>
  <compilerarg value="-XDignore.symbol.file"/>
  </javac>
- <junit printsummary="withOutAndErr" errorProperty="test.failed" failureProperty="test.failed">
+ <junit printsummary="withOutAndErr" errorProperty="test.failed" failureProperty="test.failed" fork="true">
  <sysproperty key="results.dir" value="${dir.reports.unit}"/>
  <classpath refid="test.classpath"/>
  <formatter type="xml"/>