Add UUID-based UserId generation

Author: alexander-yevsyukov

testutil/src/main/java/org/spine3/test/Tests.java

@@ -21,6 +21,7 @@
 package org.spine3.test;
 
 import com.google.protobuf.Timestamp;
+import org.spine3.base.Identifiers;
 import org.spine3.protobuf.Timestamps;
 import org.spine3.users.UserId;
 
@@ -114,6 +115,13 @@ public static UserId newUserId(String value) {
                 .build();
     }
 
+    /**
+     * Generates a new UUID-based {@code UserId}.
+     */
+    public static UserId newUserUuid() {
+        return newUserId(Identifiers.newUuid());
+    }
+
     /** The provider of current time, which is always the same. */
     public static class FrozenMadHatterParty implements Timestamps.Provider {
         private final Timestamp frozenTime;

testutil/src/test/java/org/spine3/test/TestsShould.java

@@ -84,6 +84,11 @@ public void create_UserId_by_string() {
         assertEquals(expected, userId);
     }
 
+    @Test
+    public void create_new_UUID_based_UserId() {
+        assertFalse(Tests.newUserUuid().getValue().isEmpty());
+    }
+
     @Test(expected = NullPointerException.class)
     public void do_not_accept_null_UseId_value() {
         newUserId(Tests.<String>nullRef());