Fix the typings for the arguments on the test renderer child api

[agubler]

Type: bug

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:

Fix the child typings for widgets with optional children and functional children.

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[codecov]

Codecov Report

Merging #871 (c1e2c24) into master (9bbcff9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #871   +/-   ##
=======================================
  Coverage   94.72%   94.72%           
=======================================
  Files         127      127           
  Lines        8130     8130           
  Branches     1894     1894           
=======================================
  Hits         7701     7701           
  Misses        429      429           

Impacted Files	Coverage Δ
src/testing/renderer.ts	97.32% <ø> (ø)

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