Run.dev.tests JUnit task with fork enabled to prevent hanging

[fentonreid]

The JUnit task for run.dev.tests in defects4j.build.xml has the fork attribute enabled, with fork mode set to 'once' to create a single JVM for all tests. A default timeout of 3 minutes is also specified, to stop the execution of all tests after this period.

This change allows for endless execution of relevant tests for the defects4j 'test' command with the -r flag. Without the run.dev.tests task hanging indefinitely, a problem encountered in Dockerized defects4J.