Fix race condition testing global state (#28536)

Part of #27306

test/EFCore.Design.Tests/Design/DbContextActivatorTest.cs

@@ -8,40 +8,24 @@ public class DbContextActivatorTest
     [ConditionalFact]
     public void CreateInstance_works()
     {
-        EF.IsDesignTime = false;
-
-        var result = DbContextActivator.CreateInstance(typeof(TestContext));
-
-        Assert.IsType<TestContext>(result);
-
-        Assert.True(EF.IsDesignTime);
+        Assert.IsType<TestContext>(DbContextActivator.CreateInstance(typeof(TestContext)));
     }
 
     [ConditionalFact]
     public void CreateInstance_with_arguments_works()
     {
-        EF.IsDesignTime = false;
-
-        var result = DbContextActivator.CreateInstance(
+        Assert.IsType<TestContext>(DbContextActivator.CreateInstance(
             typeof(TestContext),
             null,
             null,
-            new[] { "A", "B" });
-
-        Assert.IsType<TestContext>(result);
-
-        Assert.True(EF.IsDesignTime);
+            new[] { "A", "B" }));
     }
 
     private class TestContext : DbContext
     {
         protected override void OnConfiguring(DbContextOptionsBuilder options)
-        {
-            Assert.True(EF.IsDesignTime);
-
-            options
+            => options
                 .EnableServiceProviderCaching(false)
                 .UseInMemoryDatabase(nameof(DbContextActivatorTest));
-        }
     }
 }

test/EFCore.Design.Tests/Design/DesignTimeFlagTest.cs

@@ -0,0 +1,74 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Microsoft.EntityFrameworkCore.Design;
+
+// The tests interact with global state and should never be run in parallel
+[Collection("DesignTimeFlagTest")]
+public class DesignTimeFlagTest
+{
+    [ConditionalFact]
+    public void Operations_have_design_time_flag_set()
+    {
+        EF.IsDesignTime = false;
+
+        var handler = new OperationResultHandler();
+
+        new MockOperation<string>(handler, () =>
+        {
+            Assert.True(EF.IsDesignTime);
+            return "Twilight Sparkle";
+        });
+
+        Assert.False(EF.IsDesignTime);
+        Assert.Equal("Twilight Sparkle", handler.Result);
+    }
+
+    [ConditionalFact]
+    public void CreateInstance_sets_design_time_flag()
+    {
+        EF.IsDesignTime = false;
+
+        Assert.IsType<TestContext>(DbContextActivator.CreateInstance(typeof(TestContext)));
+
+        Assert.True(EF.IsDesignTime);
+    }
+
+    [ConditionalFact]
+    public void CreateInstance_with_arguments_sets_design_time_flag()
+    {
+        EF.IsDesignTime = false;
+
+        Assert.IsType<TestContext>(DbContextActivator.CreateInstance(
+            typeof(TestContext),
+            null,
+            null,
+            new[] { "A", "B" }));
+
+        Assert.True(EF.IsDesignTime);
+    }
+
+    private class TestContext : DbContext
+    {
+        protected override void OnConfiguring(DbContextOptionsBuilder options)
+        {
+            Assert.True(EF.IsDesignTime);
+
+            options
+                .EnableServiceProviderCaching(false)
+                .UseInMemoryDatabase(nameof(DbContextActivatorTest));
+        }
+    }
+
+    private class MockOperation<T> : OperationExecutor.OperationBase
+    {
+        public MockOperation(IOperationResultHandler resultHandler, Func<T> action)
+            : base(resultHandler)
+            => Execute(action);
+    }
+}
+
+[CollectionDefinition("DesignTimeFlagTest", DisableParallelization = true)]
+public class DesignTimeFlagTestCollection
+{
+}

test/EFCore.Design.Tests/Design/OperationExecutorTest.cs

@@ -17,24 +17,6 @@ public void Ctor_validates_arguments()
 
     public class OperationBaseTests
     {
-        [ConditionalFact]
-        public void Operations_have_design_time_flag_set()
-        {
-            EF.IsDesignTime = false;
-
-            var handler = new OperationResultHandler();
-            var result = "Twilight Sparkle";
-
-            new MockOperation<string>(handler, () =>
-            {
-                Assert.True(EF.IsDesignTime);
-                return result;
-            });
-
-            Assert.False(EF.IsDesignTime);
-            Assert.Equal(result, handler.Result);
-        }
-
         [ConditionalFact]
         public void Execute_catches_exceptions()
         {