(split from main repo) Execute unit tests

The unit tests can now be invoked from a command line
with `-test` switch, e.g.:
```
> .\build.cmd -test
```

Notable changes:
* Rename unit tests projects
* Ignore unit tests for the external projects and the VSIX

Author: RussKie

Properties/AssemblyInfo.cs

@@ -2,4 +2,4 @@
 using System.Runtime.CompilerServices;
 
 [assembly: AssemblyDescription("GitExtensions plugin for integration with Gerrit")]
-[assembly: InternalsVisibleTo("GerritTests")]
+[assembly: InternalsVisibleTo("Gerrit.Tests")]