Fix to shut down Excel at the end of every test

This fixes the issue where Excel.exe process was not shutting down at the end of every Excel node test. 

**Background**
Excel shutdown was previously triggered by the ```DynamoModel``` shutdown event. However when the ```DynamoModel``` was no longer made accessible to the node classes (due to the removal of ```DynSettings```) , Excel shutdown was later tied to the parent process exiting. However, in the case of NUnit tests, the process (Nunit) does not exit which explains why Excel does not shutdown anymore. 

This submission fixes the issue by adding an explicit call to the Excel cleanup routine ```OnProcessExit``` from the Excel test framework in ```ExcelTests.cs```

@pboyer would be great if you could take a clook. Thanks.