fix: remove global Retry(3) from InvokableTestBase to stop masking flaky tests

The global [Retry(3)] on InvokableTestBase silently retried all ~50
engine test classes, hiding intermittent failures. Remove it and apply
[Retry(3)] only to the four test classes that are genuinely timing-
sensitive (cancellation hooks, external cancellation, parallelism
validation, constraint-key scheduler concurrency).

Closes #4878

Author: thomhurst

TUnit.Engine.Tests/CancellationAfterHooksTests.cs

@@ -8,6 +8,7 @@ namespace TUnit.Engine.Tests;
 /// Validates that After hooks execute even when tests are cancelled (Issue #3882).
 /// These tests run the cancellation test scenarios and verify that After hooks created marker files.
 /// </summary>
+[Retry(3)]
 public class CancellationAfterHooksTests(TestMode testMode) : InvokableTestBase(testMode)
 {
     private static readonly string TempPath = Path.GetTempPath();

TUnit.Engine.Tests/ExternalCancellationTests.cs

@@ -17,6 +17,7 @@ namespace TUnit.Engine.Tests;
 /// See: https://github.com/Tyrrrz/CliWrap/issues/47
 /// </remarks>
 [ExcludeOn(OS.Windows)]
+[Retry(3)]
 public class ExternalCancellationTests(TestMode testMode) : InvokableTestBase(testMode)
 {
     private static readonly string TempPath = Path.GetTempPath();

TUnit.Engine.Tests/InvokableTestBase.cs

@@ -8,7 +8,6 @@
 namespace TUnit.Engine.Tests;
 
 [MethodDataSource(nameof(GetTestModes))]
-[Retry(3)]
 public abstract class InvokableTestBase(TestMode testMode)
 {
     public static IEnumerable<TestMode> GetTestModes()

TUnit.Engine.Tests/ParallelismValidationEngineTests.cs

@@ -10,6 +10,7 @@ namespace TUnit.Engine.Tests;
 /// 2. ParallelLimiter correctly limits concurrency
 /// 3. Different parallel limiters work independently
 /// </summary>
+[Retry(3)]
 public class ParallelismValidationEngineTests(TestMode testMode) : InvokableTestBase(testMode)
 {
     [Test]

TUnit.Engine.Tests/Scheduling/ConstraintKeySchedulerConcurrencyTests.cs

@@ -10,6 +10,7 @@ namespace TUnit.Engine.Tests.Scheduling;
 /// 2. No deadlocks occur under high contention
 /// 3. Tests complete within reasonable timeout
 /// </summary>
+[Retry(3)]
 public class ConstraintKeySchedulerConcurrencyTests(TestMode testMode) : InvokableTestBase(testMode)
 {
     [Test]