Add tests of client code generation components

[dougbu]

Is this a Bug or Feature request?

Enhancement that's part of #4896

Description of the problem

Have previously been manually testing the client code generation feature. Need to automate this, including tests of the tasks, the generic document tool, and the MSBuild targets.

Tasks should be relatively simple to unit test. But, the remainder will likely need functional tests that (unfortunately) write to disk. Will have a look at aspnet/DotNetTools, aspnet/EntityFrameworkCore, and aspnet/websdk to see how those repos test tools and MSBuild targets. May also discuss potential MSBuild and tool testing approaches with @pranavkm and @ryanbrandenburg.

Version of Microsoft.AspNetCore.Mvc or Microsoft.AspNetCore.App or Microsoft.AspNetCore.All

See the features/client.code.generation branch branch. That work is intended for the 2.2 Preview 3 milestone.