Fix: NUnit Test Adapter for Visual Studio doesn't work well.

[jsakamoto]

When I tried to build the solution "OOXML.Test.sln" of the commit 0d3b44d, I ran into the trouble that Visual Studio (2017) Test Explorer was empty.

The output window on Visual Studio reported what happened.

------ Discover test started ------
NUnit VS Adapter 2.0.0.0 discovering tests is started
Attempt to load assembly with unsupported test framework in  npoi\solution\Lib\.net4\NPOI.OOXML.Testcases.dll
Assembly nunit.framework, Version=2.6.3.13283, Culture=neutral, PublicKeyToken=96d09a1eb7f44a77 loaded through npoi\solution\Lib\.net4\NPOI.TestCases.dll failed. Assembly is ignored. Correct deployment of dependencies if this is an error.
Assembly nunit.framework, Version=2.6.3.13283, Culture=neutral, PublicKeyToken=96d09a1eb7f44a77 loaded through npoi\solution\Lib\.net4\OOXML4Net.Testcases.dll failed. Assembly is ignored. Correct deployment of dependencies if this is an error.
NUnit VS Adapter 2.0.0.0 discovering test is finished
========== Discover test finished: 0 found (0:00:04.9490244) ==========

Then, I resolve this problem by follow step.

• NUnitTestAdapter should keep v.2.0.0 to detect nested test assemblies ("NPOI.OOXML.Testcases.dll" derived from "NPOI.TestCases.dll". In this case, NUnitTestAdapter v.2.1+ could not work well.).
• Align all NUnit framework assembly versions to 2.6.3. (at the commit 0d3b44d, two projects referenced nunit.framework.dll v.2.6.3, but left one project referenced v.3.6.1 )