add SnowflakeId unit tests

test/DotNetCore.CAP.Test/SnowflakeIdTest.cs

+using System.Linq;
+using System.Threading.Tasks;
+using DotNetCore.CAP.Infrastructure;
+using Xunit;
+
+namespace DotNetCore.CAP.Test
+{
+    public class SnowflakeIdTest
+    {
+        [Fact]
+        public void NextIdTest()
+        {
+            var result = SnowflakeId.Default().NextId();
+
+            Assert.IsType<long>(result);
+            Assert.True(result > 0);
+            Assert.True(result.ToString().Length == long.MaxValue.ToString().Length);
+        }
+
+        [Fact]
+        public void ConcurrentNextIdTest()
+        {
+            var array = new long[1000];
+
+            Parallel.For(0, 1000, i =>
+            {
+                var id = SnowflakeId.Default().NextId();
+                array[i] = id;
+            });
+
+            Assert.True(array.Distinct().Count() == 1000);
+        }
+
+    }
+}