Add unit tests to aws-csharp

[justinvp]

This change updates the aws-csharp template to include unit tests, similar to #863 for aws-typescript, #864 for aws-python, and #866 for aws-go.

Part of #865

[justinvp]

The more idiomatic thing to do would be to have the program and test projects SxS:

proj/
├── Pulumi.yaml
├── Program.cs
└── proj.csproj
tests/
├── InfraTests.cs
└── Tests.csproj

But we can't easily do that from a template. If we structured the template like this, you wouldn't be able to run pulumi up after pulumi new because the program would live in a subdirectory. You'd have to cd into the program's subdirectory first.

I think we want to keep it such that you can run pulumi up immediately after pulumi new, without having to change directories. So we nest the test project:

proj/
├── Pulumi.yaml
├── Program.cs
├── proj.csproj
└── tests/
    ├── InfraTests.cs
    └── Tests.csproj

To do so, we have to exclude the source files in the nested project here.

We have a similar issue with parameterized providers, where pulumi package add generates the sdks in a sdks subdirectory, and we ask users to add <DefaultItemExcludes> for that. For that reason, I've included excludes for both sdks/ and tests/ by default.