build(deps): bump xunit from 2.5.3 to 2.6.1

[filzrev]

What's included in this PR

• Update xunit version from 2.5.3 to 2.6.1
• Fix problems that cause StackOverflowException when upgraded xUnit package to 2.6.0 or later.

Background
It seems xUnit don't support Assert.Equal for object that contains circular reference.
For example. When executing following code. Test process crash with Stackoverflow. (Tested with version 2.5.3 / 2.6.0 / 2.6.1)

 var dict = new Dictionary<string, object>
 {
 };
 dict["Ref"] = dict; // Set circular reference to self
 Assert.Equals(dict, dict["Ref"]); // Cause StackOverflowException

I don't know what changes of xUnit 2.6.0 cause this problem Release Note
But it seems some assertion library changes cause above circular reference problem.

So I've changed Assert.Equals assertion logics to Assert.Same that test object reference equity.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

see 1 file with indirect coverage changes

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[yufeih]

Thank you @filzrev !