Add basic test

src/tasks/tests/ConvertDllsToWebCilTests.cs

@@ -0,0 +1,53 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+using Moq;
+using System.Collections.Generic;
+using System.IO;
+using Xunit;
+
+namespace Microsoft.NET.Sdk.WebAssembly.Tests
+{
+    public static class ConvertDllsToWebCilTests
+    {
+        private static ConvertDllsToWebCil task;
+        private static List<BuildErrorEventArgs> errors;
+
+        static ConvertDllsToWebCilTests()
+        {
+            task = new ConvertDllsToWebCil();
+            var buildEngine = new Mock<IBuildEngine>();
+            errors = new List<BuildErrorEventArgs>();
+            buildEngine.Setup(x => x.LogErrorEvent(It.IsAny<BuildErrorEventArgs>())).Callback<BuildErrorEventArgs>(e => errors.Add(e));
+            task.BuildEngine = buildEngine.Object;
+        }
+
+        [Fact]
+        public static void TestEmptyInput()
+        {
+            string input = Path.Combine(Path.GetTempPath(), Path.GetRandomFileName() + ".dll");
+
+            try
+            {
+                File.Create(input).Dispose();
+
+                task.Candidates = new ITaskItem[] { new TaskItem(input) };
+                task.IsEnabled = true;
+                task.OutputPath = task.IntermediateOutputPath = Path.GetTempPath();
+
+                bool result = task.Execute();
+
+                Assert.False(result);
+                Assert.Single(errors);
+                Assert.Contains(input, errors[0].Message);
+            }
+            finally
+            {
+                File.Delete(input);
+            }
+        }
+
+    }
+}

src/tasks/tests/Microsoft.NET.Sdk.WebAssembly.Pack.Tasks.Tests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>$(NetCoreAppToolCurrent)</TargetFramework>
+    <Nullable>enable</Nullable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Build.Utilities.Core" Version="$(MicrosoftBuildFrameworkVersion)" />
+    <PackageReference Include="Moq" Version="$(MoqVersion)" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Microsoft.NET.Sdk.WebAssembly.Pack.Tasks\Microsoft.NET.Sdk.WebAssembly.Pack.Tasks.csproj" />
+  </ItemGroup>
+
+</Project>