Adapt test for git plugin 5.0

Git plugin 5.0 will require Java 11 and will add symbols to improve
the Pipeline syntax editing experience.  The test being modified in
this pull request checks the specific syntax of the Pipeline snippet
syntax generator.  Adapt the test to check for current syntax with git
plugin 4.x releases and for new syntax with any other releases.

jenkinsci/git-client-plugin#939 is the git
client plugin pull request that prepares git client plugin 4.0.0.
It will require Java 11 and will upgrade from JGit 5.13.1 to JGit 6.4.0.

jenkinsci/bom#1619 is the bom draft pull request
that confirms git client plugin 4.0.0 pre-release works in the plugin bom.

jenkinsci/git-plugin#1367 is the git plugin pull
request that prepares git plugin 5.0.0.  It will require Java 11.

jenkinsci/bom#1624 is the bom draft pull request
that tests git plugin 5.0.0 works in the plugin bom.

jenkinsci/git-plugin#1373 is the git plugin pull
request that relies on jenkinsci/git-plugin#1367
and adds symbols to the git plugin.

jenkinsci/bom#1625 is the bom draft pull request
that tests git plugin 5.0.0 with added symbols works in the plugin bom.
It detected this test failure when run with git plugin 5.0.0 pre-release.

Author: MarkEWaite

src/test/java/org/jenkinsci/plugins/workflow/libs/LibraryStepTest.java

@@ -85,7 +85,11 @@ public class LibraryStepTest {
             null, null, Collections.<GitSCMExtension>emptyList())));
         s.setChangelog(false);
         r.assertEqualDataBoundBeans(s, stepTester.configRoundTrip(s));
-        snippetizerTester.assertRoundTrip(s, "library changelog: false, identifier: 'foo@master', retriever: legacySCM([$class: 'GitSCM', branches: [[name: '${library.foo.version}']], extensions: [], userRemoteConfigs: [[url: 'https://nowhere.net/']]])");
+        if (r.jenkins.get().getPlugin("git").getWrapper().getVersion().startsWith("4.")) {
+            snippetizerTester.assertRoundTrip(s, "library changelog: false, identifier: 'foo@master', retriever: legacySCM([$class: 'GitSCM', branches: [[name: '${library.foo.version}']], extensions: [], userRemoteConfigs: [[url: 'https://nowhere.net/']]])");
+        } else {
+            snippetizerTester.assertRoundTrip(s, "library changelog: false, identifier: 'foo@master', retriever: legacySCM(scmGit(branches: [[name: '${library.foo.version}']], extensions: [], userRemoteConfigs: [[url: 'https://nowhere.net/']]))");
+        }
     }
 
     @Test public void vars() throws Exception {