How to test:

Prerequisites
Identical or simliar installations as seen in Versions

Versions
Windows: 7
Visual Studio: 2017 Enterprise
.NET: 4.5.2
NUnit: 2.6.4
NUnit Adapter: NUnit 2 Test Adapter 2.1.1
Excel: Exel 2016

Reproduction:

• Open solution and build to discover unit tests.
• Ensure any currently running instances of Excel are fully closed.
• Run both the tests without Code Coverage. (NUnit 2 Test Adapter needed for NUnit test)

• Observe them both executing just fine.

• Run one of them with Code Coverage.

• Hangs indefinitely and never closes instance.

• To prepare for second test, restart Visual Studio and ensure that the background Excel instance is closed manually. Do this through Task Manager and merely look for 'EXCEL.EXE'.
• Now attempt to run the second test and observe the same behavior as seen in first test.