Assertion Templates

[matt-gadd]

Type: feature

The following has been addressed in the PR:

• All code has been formatted with prettier as per the readme code style guidelines
• Unit or Functional tests are included in the PR

Description:
Asserting full render functions can be brittle and end up encapsulating lots of logic in tests. While asserting only portions of a render function does not potentially cover everything (because a render function is the smallest unit).

To solve the above problems, Assertion Templates allow the user to always assert against a full render, while allowing them to easily modify the assertion themselves per scenario.

The AssertionTemplate uses the existing selector api used elsewhere in harness, and supports an additional magic ~key selector which is erased on render, this allows the user to select nodes in the AssertionTemplate, without the key being in the actual render result itself.

The api for AssertionTemplate is very simple with the following api's:

• getProperty - gets a property on a selected node
• setProperty - sets a property on a selected node
• getChildren - gets the children on a selected node
• setChildren - sets the children on a selected node

AssertionTemplates are always immutable, setting a value will always result in a new AssertionTemplate being returned. This allows you to use assertions that build on top of other assertions.

Resolves #190