Nunit1032 - consider dispose of a type having explicit interface implementation

[matode]

In case a class does inherit from IDisposable, but Dispose is implemented with explicit interface implementation (void IDisposable.Dispose()), then the Dispose call in TearDown method is not considered by NUnit1032.

This change considers casting of given type to IDisposable and doesn't report a finding in such case

[matode]

@dotnet-policy-service agree

[mikkelbu]

LGTM. I considering if we could make the code in GetIdentifier more general to e.g. also support cases such like

private DummyExplicitDisposable field2;
...
[OneTimeTearDown]
public void Dispose()
{
    IDisposable x = field2;
    x.Dispose();
}

But I think this is already an improvement, and then we can always improve on this later on

[mikkelbu]

Thanks for the contribution @matode. I'll merge it now