Change list() type to match that of super in test

JGitHealthCheckTest failed to compile due to the super's list() return
type having changed type. Update the test's instance to match the
updated type.

Bug: Issue 15848
Change-Id: Ib9baab3066625edd24faba7be6dd7dd705554149
diff --git a/src/test/java/com/googlesource/gerrit/plugins/healthcheck/JGitHealthCheckTest.java b/src/test/java/com/googlesource/gerrit/plugins/healthcheck/JGitHealthCheckTest.java
index 72259a5..5c1fd73 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/healthcheck/JGitHealthCheckTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/healthcheck/JGitHealthCheckTest.java
@@ -32,7 +32,7 @@
 import com.googlesource.gerrit.plugins.healthcheck.check.JGitHealthCheck;
 import java.io.IOException;
 import java.util.Collections;
-import java.util.SortedSet;
+import java.util.NavigableSet;
 import org.eclipse.jgit.errors.RepositoryNotFoundException;
 import org.eclipse.jgit.lib.CommitBuilder;
 import org.eclipse.jgit.lib.Constants;
@@ -119,8 +119,8 @@
       }
 
       @Override
-      public SortedSet<Project.NameKey> list() {
-        return Collections.emptySortedSet();
+      public NavigableSet<Project.NameKey> list() {
+        return Collections.emptyNavigableSet();
       }
     };
   }