Configure fork=once so that TRACE logs can be gathered more easily

We currently run our testsuite with fork=none which causes us some small issues:

• First, we're exposed to whatever system properties Maven might use internally which could impact our build.
• Secondly, it makes it difficult to have per-project log4j settings that are not hardcoded in the source code.

So, what is suggested here is that we switch to fork=once so that each module's testsuite runs in a different JVM. The overhead of this is apparently pretty small (needs double checking).

In terms of logging, this potentially enables each module to have its own 'trace-enabling' profile pointing to its own TRACE enabled log4j file, making it quite easy for each module to define what TRACE is relevant for each. For example:

<project> 
  <profiles> 
    <profile> 
      <id>trace-tests</id> 
      <build> 
        <plugins> 
          <plugin> 
            <artifactId>maven-surefire-plugin</artifactId> 
            <configuration> 
              <systemPropertyVariables> 
                <log4j.configuration>${basedir}/src/test/trace/log4j.xml</log4j.configuration> 
              </systemPropertyVariables> 
            </configuration> 
          </plugin> 
        </plugins> 
      </build> 
    </profile> 
  </profiles> 
</project>

This has another advantage which is allowing us to override the log output directory selectively which is handy in a CI environment such as CloudBees:

• First, set the logs location to be dependant on another system property, i.e.:

  location=${log4j.outputDirectory}/infinispan.log

• And then, selectively change it:

  <systemPropertyVariables> 
  <log4j.configuration>${basedir}/src/test/trace/log4j.xml</log4j.configuration> 
  <log4j.outputDirectory>${project.build.directory}/test-logs</log4j.outputDirectory> 
  </systemPropertyVariables>