MegaIssue: testing utilities

[martinbonnin]

This is a high level umbrella issue for issues related to testing utilities.

There are currently 2 main ways to obtain Operation.Data instances for testing:

1. calling the data class constructor directly.
2. capturing a JSON and then parsing it into Operation.Data instances.

Solution 1. is cumbersome because there are a lot of parameters to pass. Also in polymorphic cases, we add __typename automatically and figuring out what __typename to pass in the constructor can be confusing.

Solution 2. is cumbersome because editing JSON isn't fun. There's no autocomple/IDE support and the JSON are quite large. Also, if for some reason, the JSON shape isn't the expected one, parsing will just fail.

Also, none of these solutions allow automatically faking field values. Every field must be explicitely provided.

Data Builders attempt to fix those issues by providing a DSL to create fake Operation.Data instances. They suffer from some limitations though:

• the FakeResolver API isn't the most flexible when it comes to faking values in cental place (Have a way to specify a global custom FakeResolver for data builders #4435)
• using aliases is a bit awkward and non-typesafe
• they are always generated in the main source set (Allow for enabling data builders only in test flavours  #5257)
• the generated code can become really large ("Method too large" error during compilation of schema module #6229)

Now that we have execution algorithms, we could use them to provide a "true" schema based testing experience. This is also what the web client is doing at the moment, see https://www.youtube.com/watch?v=TCp3P5gAacA

Linked issues:

• Allow for enabling data builders only in test flavours  #5257
• Have a way to specify a global custom FakeResolver for data builders #4435
• Generate models with default parameter values #4821
• Explore the idea of marking the Data Builder DSL with @DslMarker to make it harder to make mistakes when trying to generate deeply nested types #5185
• Name clash / redeclaration using data builders #5537
• "Method too large" error during compilation of schema module #6229