Comprehensive tests for step graphs

[dirkgr]

Test cases:

• [ ] Direct step dependency
• [ ] Step dependency through a list/dict/set/tuple
• [ ] Step dependency through a complex (FromParams) object
• [ ] Anonymous step dependency (i.e., when the dependency doesn't have a name and is just specified directly)
• [ ] Reference to a step by unique id instead of name
• [ ] Reference to a step by unique id which is also an anonymous step dependency
• [ ] Circular dependencies (should fail cleanly)
• [ ] Dependency where the types don't match (should fail cleanly)

All of these should work

• [ ] with a config file
• [ ] with Tango-as-a-library (where it makes sense)

All of these should work with steps that are

• [ ] cacheable
• [ ] not cacheable
• [ ] deterministic
• [ ] not deterministic

In the deterministic case, we should make sure the generated unique ids are identical when the same thing runs a second time. In the non-deterministic case, we should make sure they are not.