Bump NUnit from 2.6.4 to 3.12.0

Bumps [NUnit](https://github.com/nunit/nunit) from 2.6.4 to 3.12.0.
- [Release notes](https://github.com/nunit/nunit/releases)
- [Changelog](https://github.com/nunit/nunit/blob/master/CHANGES.md)
- [Commits](https://github.com/nunit/nunit/commits/v3.12)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

src/OrigoDB.Core.UnitTests/packages.config

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
 <packages>
   <package id="FakeItEasy" version="1.25.3" targetFramework="net40" />
-  <package id="NUnit" version="2.6.4" targetFramework="net40" />
+  <package id="NUnit" version="3.12.0" targetFramework="net40" />
 </packages>