Verify semantic symbols in using expression tests

[Copilot]

Two tests discarded a lazy symbol-verification iterator, so their assertions never ran. The helper also applies only to using declarations, while these tests cover using expressions.

• Semantic assertions: Verify that the identifier expression binds to its declared local and that the object-creation expression resolves to MyManagedType.
• Diagnostics: Verify clean compilation after removing unused using System; directives.

Assert.Equal(declaredSymbol, model.GetSymbolInfo(usingStatement.Expression).Symbol);

• Fixes Two tests are useless because they do not actually verify anything #85841

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