Support testing widgets with nested typed children

[agubler]

Type: feature

The following has been addressed in the PR:

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

Description:

Provides an API to support testing widgets with complex, nested typed children, for example:

interface TypedChildren {
    headers: {
        headerA: (value: string, num: number) => RenderResult    
    },
    cells: {
        columnA: (value: string) => RenderResult
    },
    footer: RenderResult
}

Currently the test renderer's child API enables consumers to instruct the renderer to resolve children to one level, renderer.child(WrappedWidget, 'child', 'params', 'for', 'renderer'); this change adds an additional overload that enables users to provide params to resolve multiple children, including nested children. The function is typed to require all functional children to have params returned and the params are typed for the functional child. Using the example child interface above, this would look like:

renderer.child(WrappedWidget, (params) => {
    return {
        headers: {
            headerA: params(['text', 12345])
        },
        cells: {
            columnA: params(['text'])
        }
    };
}); 

Note: The params function was required for typings, otherwise the parameters for the functions were typed as tuples not arrays so need explicit casting i.e. ['text'] as [string] - using an injected function seemed to be a more user friendly approach.

Includes fixes required for parsing nested children in assertRender

Resolves #897

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[codecov]

Codecov Report

Merging #898 (7d8a7a1) into master (36aae1a) will increase coverage by 0.04%.
The diff coverage is 98.66%.

@@            Coverage Diff             @@
##           master     #898      +/-   ##
==========================================
+ Coverage   94.54%   94.59%   +0.04%     
==========================================
  Files         127      127              
  Lines        8121     8135      +14     
  Branches     1896     1899       +3     
==========================================
+ Hits         7678     7695      +17     
+ Misses        443      440       -3     

Impacted Files	Coverage Δ
src/testing/assertRender.ts	98.18% <96.66%> (+2.31%)	⬆️
src/testing/decorate.ts	98.36% <100.00%> (+0.34%)	⬆️
src/testing/renderer.ts	97.36% <100.00%> (+0.04%)	⬆️

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