Find a reasonable way to test incremental compilation in msbuild tasks

[aspnet-hello]

From @javiercn on Friday, December 15, 2017 3:46:30 PM

We need to find a reasonable way to write tests that ensure that we don't break incremental compilation in our msbuild tasks.

See as an example
https://github.com/dotnet/sdk/blob/73fb22a07091585380ead0bcbcdc38839a0266e8/test/Microsoft.NET.Build.Tests/GivenThatWeWantToControlGeneratedAssemblyInfo.cs#L122-L194

Copied from original issue: aspnet/FileSystem#309

[muratg]

@javiercn Do you remember why you logged this in FileSystem repo originally?