Support auto populate TestSpec based on input

[mingwli]

Currently in our Loc Transformer, we need to frequently update our skeleton structure, especially in an unstable developing stage.
This brings us much effort updating the output test data.

One suggestion is auto populate test spec output:

• only provide input data
• add a flag that instructs test framework to auto update the test spec with expected output
• further customized formatting can be applied to the output graph

cc @yufeih @live1206