Fix #2581: Copy WarningsAsErrors setting to generated project

Add WarningsAsErrors to SettingsWeWantToCopy array in CsProjGenerator
so that NuGet restore warnings configured in the source project are
preserved in the generated benchmark project and fail the build.

Added test WarningsAsErrorsSettingGetsCopied() to verify the fix.

Author: Shrinidhi203

src/BenchmarkDotNet/Toolchains/CsProj/CsProjGenerator.cs

@@ -38,6 +38,7 @@ public class CsProjGenerator : DotNetCliGenerator, IEquatable<CsProjGenerator>
             "UserSecretsId",
             "EnablePreviewFeatures",
             "RuntimeHostConfigurationOption",
+            "WarningsAsErrors",
         }.ToImmutableArray();
 
         public string RuntimeFrameworkVersion { get; }

tests/BenchmarkDotNet.Tests/CsProjGeneratorTests.cs

@@ -181,6 +181,28 @@ public void RuntimeHostConfigurationOptionIsCopied()
             Assert.Equal("Microsoft.NET.Sdk", sdkName);
         }
 
+        [Fact]
+        public void WarningsAsErrorsSettingGetsCopied()
+        {
+            const string withWarningsAsErrors = @"
+<Project Sdk=""Microsoft.NET.Sdk"">
+  <PropertyGroup>
+    <WarningsAsErrors>NU1102;NU1603</WarningsAsErrors>
+  </PropertyGroup>
+</Project>
+";
+            var sut = new CsProjGenerator("netcoreapp3.1", null, null, null, true);
+
+            var xmlDoc = new XmlDocument();
+            xmlDoc.LoadXml(withWarningsAsErrors);
+            var (customProperties, sdkName) = sut.GetSettingsThatNeedToBeCopied(xmlDoc, TestAssemblyFileInfo);
+
+            AssertCustomProperties(@"<PropertyGroup>
+  <WarningsAsErrors>NU1102;NU1603</WarningsAsErrors>
+</PropertyGroup>", customProperties);
+            Assert.Equal("Microsoft.NET.Sdk", sdkName);
+        }
+
         [Fact]
         public void TheDefaultFilePathShouldBeUsedWhenAnAssemblyLocationIsEmpty()
         {