Editor: Add tests for DefaultBlockAppender

[gziolo]

Description

Follow-up for #3623.

As discussed in the referenced PR, I'm adding tests for newly introduced DefaultBlockAppender component.

How Has This Been Tested?

npm run test-unit

Types of changes

Adds tests.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows has proper inline documentation.

[youknowriad]

Awesome Thanks

[codecov]

Codecov Report

Merging #3638 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #3638     +/-   ##
========================================
+ Coverage   36.69%   36.8%   +0.1%     
========================================
  Files         268     268             
  Lines        6649    6649             
  Branches     1201    1201             
========================================
+ Hits         2440    2447      +7     
+ Misses       3557    3550      -7     
  Partials      652     652

Impacted Files	Coverage Δ
editor/components/default-block-appender/index.js	87.5% <ø> (+87.5%)	⬆️

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