Flaky test: RichTextBox_Text_GetWithHandle_ReturnsExpected

[RussKie]

Problem description:

Few variations, but generally the failing point is this:

    System.Windows.Forms.Tests.RichTextBoxTests.RichTextBox_Text_GetWithHandle_ReturnsExpected [FAIL]
      Assert.Equal() Failure
                � (pos 0)
      Expected: a?b
      Actual:   ?b
                � (pos 0)
      Stack Trace:
        /_/src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/RichTextBoxTests.cs(6918,0): at System.Windows.Forms.Tests.RichTextBoxTests.RichTextBox_Text_GetWithHandle_ReturnsExpected()

The test started failing repeatedly recently, in x86 and only on the public CI.

Observed:

• Reported Fix flaky test #4342 (comment)
• https://dev.azure.com/dnceng/public/_build/results?buildId=913908&view=results

[JeremyKuhne]

Assert.Equal() Failure
          ↓ (pos 0)
Expected: a b
Actual:    b
          ↑ (pos 0)

[dreddy-work]

Dupe of #6609