add tests for JsonReporter class

test/coverlet.core.tests/Reporters/JsonReporterTests.cs

@@ -0,0 +1,16 @@
+using Xunit;
+
+namespace Coverlet.Core.Reporters.Tests
+{
+    public class JsonReporterTests
+    {
+        [Fact]
+        public void TestFormat()
+        {
+            JsonReporter reporter = new JsonReporter();
+            CoverageResult result = new CoverageResult { Data = new Data() };
+
+            Assert.Equal("{\n}", reporter.Format(result));
+        }
+    }
+}