Bump jenkins-test-harness from 2.55 to 2.56

Bumps [jenkins-test-harness](https://github.com/jenkinsci/jenkins-test-harness) from 2.55 to 2.56.
- [Release notes](https://github.com/jenkinsci/jenkins-test-harness/releases)
- [Commits](jenkinsci/jenkins-test-harness@jenkins-test-harness-2.55...jenkins-test-harness-2.56)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

pom.xml

@@ -53,7 +53,7 @@
     <!-- Should only need to override these if using timestamped snapshots (but better to use Incrementals instead): -->
     <jenkins-core.version>${jenkins.version}</jenkins-core.version>
     <jenkins-war.version>${jenkins.version}</jenkins-war.version>
-    <jenkins-test-harness.version>2.55</jenkins-test-harness.version>
+    <jenkins-test-harness.version>2.56</jenkins-test-harness.version>
     <hpi-plugin.version>3.9</hpi-plugin.version>
     <stapler-plugin.version>1.17</stapler-plugin.version>
     <java.level>you-must-override-the-java.level-property</java.level>