Fix unreliable unit test

Fixes #389

pgpainless-core/src/main/java/org/pgpainless/key/modification/secretkeyring/SecretKeyRingEditor.java

@@ -83,6 +83,12 @@ public class SecretKeyRingEditor implements SecretKeyRingEditorInterface {
         this.referenceTime = referenceTime;
     }
 
+    @Nonnull
+    @Override
+    public Date getReferenceTime() {
+        return referenceTime;
+    }
+
     @Override
     public SecretKeyRingEditorInterface addUserId(
             @Nonnull CharSequence userId,

pgpainless-core/src/main/java/org/pgpainless/key/modification/secretkeyring/SecretKeyRingEditorInterface.java

@@ -29,6 +29,14 @@ import org.pgpainless.util.selection.userid.SelectUserId;
 
 public interface SecretKeyRingEditorInterface {
 
+    /**
+     * Return the editors reference time.
+     *
+     * @return reference time
+     */
+    @Nonnull
+    Date getReferenceTime();
+
     /**
      * Add a user-id to the key ring.
      *