Throw GitException as expected in GitClientTest

When command line git does not support remote symbolic references, the
test intentionally exits early.  In order to pass the test, it needs
to throw the expected exception instead of simply returning. Throw the
expected exception.

Allows tests to run successfully on CentOS 7 with git 1.8.3.

src/test/java/org/jenkinsci/plugins/gitclient/GitClientTest.java

@@ -2569,7 +2569,7 @@ public void testgetRemoteSymbolicReferences_with_non_default_HEAD() throws Excep
     @Test(expected = GitException.class)
     public void testgetRemoteSymbolicReferences_URI_Syntax() throws Exception {
         if (!CLI_GIT_SUPPORTS_SYMREF) {
-            return;
+            throw new GitException("Skipping JGit tests in testgetRemoteSymbolicReferences_URI_Syntax");
         }
         gitClient.getRemoteSymbolicReferences("error: invalid repo URL", Constants.HEAD);
     }