Fix unit tests assertion parameters for expected and actual values

## Expected behavior
In unit tests, `Assert` methods such as `Assert.AreEqual(value, otherValue)` should have the first parameter be the expected value, second parameter the actual value.

## Current behavior
Some correct usage, mostly backwards. [Example](https://github.com/CommunityToolkit/dotnet/blob/main/tests/CommunityToolkit.Mvvm.UnitTests/Test_ICommandAttribute.cs#L20)

Start with tests/CommunityToolkit.Mvvm.UnitTests/Test_ICommandAttribute.cs and inspect the Assert calls referenced by the issue. Review the unit-test suite for reversed expected and actual parameters, then run the relevant tests to confirm the assertion ordering is corrected throughout.

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