Don't suggest FAA0002 for xunit 3's Assert.Skip... usage

Currently when using conditional tests with xUnit 3's Assert.Skip... I get the notification that I should use the Fluent Assertions alternative, however no such alternative exists.

```csharp
[Fact]
public void TestsSomething()
{
Assert.SkipWhen(true, reason: "skip this when true"); // <- this should not raise an analyzer FAA0002 issue.

// Arrange
// ...

// Act
var result = true;

// Assert
result.Should().BeFalse();
}
```

Locate the implementation and tests for diagnostic FAA0002, then reproduce the example using xUnit 3's Assert.SkipWhen. Update the analyzer behavior so Assert.Skip... calls do not raise FAA0002, and add a regression test confirming the diagnostic is absent while applicable Fluent Assertions guidance remains covered.

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