fix: serialize integration tests to prevent file locking

Add XUnit collection definitions and [Collection] attributes to all
integration test classes to prevent parallel execution. This fixes the
file locking issue where multiple tests try to build the same project
simultaneously, causing "cannot access file ... being used by another
process" errors.

Test collections:
- SimpleProject: Tests for SimpleProject.csproj run sequentially
- MultiTargetProject: Tests for MultiTargetProject.csproj run sequentially
- DisabledProject: Tests for DisabledProject.csproj run sequentially
- Configuration: Configuration tests run sequentially

Tests in different collections can still run in parallel, but tests
within a collection are serialized to avoid file contention.

This fixes the CS2012 error: "SimpleProject.dll could not be opened for
writing" that occurred during local Visual Studio test runs.

Author: claude

tests/CycloneDX.MSBuild.Tests/IntegrationTests/ConfigurationTests.cs

@@ -7,6 +7,7 @@ namespace CycloneDX.MSBuild.Tests.IntegrationTests;
 /// <summary>
 /// Tests for various configuration options.
 /// </summary>
+[Collection("Configuration")]
 public class ConfigurationTests : IDisposable
 {
     private readonly string _projectPath;

tests/CycloneDX.MSBuild.Tests/IntegrationTests/DisabledProjectTests.cs

@@ -7,6 +7,7 @@ namespace CycloneDX.MSBuild.Tests.IntegrationTests;
 /// <summary>
 /// Integration tests for projects with SBOM generation disabled.
 /// </summary>
+[Collection("DisabledProject")]
 public class DisabledProjectTests : IDisposable
 {
     private readonly string _projectPath;

tests/CycloneDX.MSBuild.Tests/IntegrationTests/MultiTargetProjectTests.cs

@@ -7,6 +7,7 @@ namespace CycloneDX.MSBuild.Tests.IntegrationTests;
 /// <summary>
 /// Integration tests for multi-target framework projects.
 /// </summary>
+[Collection("MultiTargetProject")]
 public class MultiTargetProjectTests : IDisposable
 {
     private readonly string _projectPath;

tests/CycloneDX.MSBuild.Tests/IntegrationTests/SimpleProjectTests.cs

@@ -7,6 +7,7 @@ namespace CycloneDX.MSBuild.Tests.IntegrationTests;
 /// <summary>
 /// Integration tests for simple single-target projects.
 /// </summary>
+[Collection("SimpleProject")]
 public class SimpleProjectTests : IDisposable
 {
     private readonly string _projectPath;

tests/CycloneDX.MSBuild.Tests/IntegrationTests/TestCollections.cs

@@ -0,0 +1,33 @@
+using Xunit;
+
+namespace CycloneDX.MSBuild.Tests.IntegrationTests;
+
+/// <summary>
+/// XUnit collection definitions to control test parallelization.
+/// Tests in the same collection run sequentially to avoid file locking issues.
+/// </summary>
+
+// SimpleProject tests - serialize to avoid file locking on SimpleProject.dll
+[CollectionDefinition("SimpleProject")]
+public class SimpleProjectCollection
+{
+}
+
+// MultiTargetProject tests - serialize to avoid file locking
+[CollectionDefinition("MultiTargetProject")]
+public class MultiTargetProjectCollection
+{
+}
+
+// DisabledProject tests - serialize to avoid file locking
+[CollectionDefinition("DisabledProject")]
+public class DisabledProjectCollection
+{
+}
+
+// Configuration tests can run in parallel with project-specific tests
+// but serialize among themselves
+[CollectionDefinition("Configuration")]
+public class ConfigurationCollection
+{
+}