Unstable unit test: System.Windows.Forms.Tests.TextBoxBaseTests.TextBoxBase_Undo_CanUndo_Success

Related #11227

### Test

System.Windows.Forms.Tests.TextBoxBaseTests.TextBoxBase_Undo_CanUndo_Success

### Issue description

The case failed in [PR build](https://dev.azure.com/dnceng-public/public/_build/results?buildId=712790&view=ms.vss-test-web.build-test-results-tab&runId=17822834&resultId=110643&paneView=debug), the failed scenario:

- TextBoxBase_Undo_CanUndo_Success

**Error Message:**

- **In X86 unit:**

- Assert.Equal() Failure: Strings differ
↓ (pos 0)
Expected: "bcxt"
Actual: "xt"
↑ (pos 0)

**Stack trace:**
```
at System.Windows.Forms.Tests.TextBoxBaseTests.TextBoxBase_Undo_CanUndo_Success() in /_/src/System.Windows.Forms/tests/UnitTests/TextBoxBaseTests.cs:line 7152
at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
```