Do you use Pax-Exam-Karaf or do you use plain Pax-Exam, if it's Pax-Exam-Karaf use the following for your configuration:
KarafDistributionOption.editConfigurationFilePut("etc/org.ops4j.pax.web.cfg",
"org.osgi.service.http.port", "alternative-port"),
if it's plain Pax-Exam:
systemProperty("org.osgi.service.http.port").value("8181")
You might also take a look on how Karaf does it's regression testing at Karaf Regression Test or take a look at the pax-web itests at Pax-Web Integration Tests