Support inline yaml programs

[danielrbradley]

• Add a variation of PulumiTest which doesn't take a source, but instead takes a struct describing a Pulumi YAML program.
• Add supporting methods for modifying the Pulumi YAML program such as pt.AddResource(name string, resource Resource) and pt.UpdateResource(name string, resource Resource) or pt.UpdatePath(path string, value any).
• Consider creating a new PulumiTest variation for inline programs which extends the base PulumiTest with the additional YAML-specific methods.