[RealJenkinsRule] Switching hardcoded IPv4 address to loopback address

[PereBueno]

As described in #711.

This change removes the hardcoded 127.0.0.1 address used in RealJenkinsRule and replaces it with the real loopback address, so it will use the IPv4 loopback address, 127.0.0.1, or the IPv6 loopback address, ::1 (in long format, so 0:0:0:0:0:0:0:1).

Regardless address can be overriden via withHttpListenAddress it makes sense to have default address use the right protocol so that method is used only for specific cases.

This change won't affect any test using getUrl() method, as that method returns an URL pointing to localhost so should be version-agnostic.

Testing done

Check running tests using snapshot:
With IPv6

mvn clean test -Djava.net.preferIPv6Addresses=true -Dtest=[......]
...
JENKINS_HOME=[..] SUREFIRE_FORK_NUMBER=1 java -DRealJenkinsRule.location=file:[PATH]/.m2/repository/org/jenkins-ci/main/jenkins-test-harness/999999-SNAPSHOT/jenkins-test-harness-999999-SNAPSHOT.jar [...] --httpPort=0 --httpListenAddress=0:0:0:0:0:0:0:1 --prefix=/jenkins

With IPv4

mvn clean test -Djava.net.preferIPv6Addresses=false -Dtest=[......]
...
JENKINS_HOME=[..] SUREFIRE_FORK_NUMBER=1 java -DRealJenkinsRule.location=file:[PATH]/.m2/repository/org/jenkins-ci/main/jenkins-test-harness/999999-SNAPSHOT/jenkins-test-harness-999999-SNAPSHOT.jar [...] --httpPort=0 --httpListenAddress=127.0.0.1 --prefix=/jenkins

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