Add integration tests to the `core/integration`

[wingyplus]

After help working on PHP SDK. We need integration tests in core/integration.

Steps are:

1. Add core/integration/module_dotnet_test.go.
2. Add DotNetSuite type to the file.
3. Add TestDotNet to the file.
4. Add sample func (DotNetSuite) TestInit(context.Context, t *testing.T) and run dagger call test specific --run=TestDotNet should works.