Snippets: If snippet and use of LSP's SnippetExpander

src/EditorFeatures/CSharpTest/Completion/CompletionProviders/Snippets/AbstractCSharpLSPSnippetTests.cs

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+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#nullable disable
+
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis.Completion;
+using Microsoft.CodeAnalysis.Editor.CSharp.UnitTests.Completion.CompletionProviders;
+using Roslyn.Test.Utilities;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.Test.Utilities.Snippets
+{
+    public abstract class AbstractCSharpLSPSnippetTests : AbstractCSharpCompletionProviderTests
+    {
+        protected override async Task VerifyCustomCommitProviderWorkerAsync(string codeBeforeCommit, int position, string itemToCommit, string expectedLSPSnippet, SourceCodeKind sourceCodeKind, char? commitChar = null)
+        {
+            using var workspaceFixture = GetOrCreateWorkspaceFixture();
+            var workspace = workspaceFixture.Target.GetWorkspace();
+
+            // Set options that are not CompletionOptions
+            workspace.SetOptions(WithChangedNonCompletionOptions(workspace.Options));
+
+            var document1 = workspaceFixture.Target.UpdateDocument(codeBeforeCommit, sourceCodeKind);
+            await VerifyCustomCommitProviderLSPSnippetAsync(document1, position, itemToCommit, expectedLSPSnippet, commitChar);
+
+            if (await CanUseSpeculativeSemanticModelAsync(document1, position))
+            {
+                var document2 = workspaceFixture.Target.UpdateDocument(codeBeforeCommit, sourceCodeKind, cleanBeforeUpdate: false);
+                await VerifyCustomCommitProviderLSPSnippetAsync(document2, position, itemToCommit, expectedLSPSnippet, commitChar);
+            }
+        }
+
+        private async Task VerifyCustomCommitProviderLSPSnippetAsync(Document document, int position, string itemToCommit, string expectedLSPSnippet, char? commitChar = null)
+        {
+            var service = GetCompletionService(document.Project);
+            var completionList = await GetCompletionListAsync(service, document, position, CompletionTrigger.Invoke);
+            var items = completionList.Items;
+
+            Assert.Contains(itemToCommit, items.Select(x => x.DisplayText), GetStringComparer());
+            var completionItem = items.First(i => CompareItems(i.DisplayText, itemToCommit));
+            var commit = await service.GetChangeAsync(document, completionItem, commitChar, CancellationToken.None);
+            var generatedLSPSnippet = commit.LSPSnippet;
+            AssertEx.EqualOrDiff(expectedLSPSnippet, generatedLSPSnippet);
+        }
+    }
+}