Editor: Display an item in Table of Contents when only one heading added

[gziolo]

Description

You need to add at least 2 heading to the document to see them listed in the table of contents. This PR tries to fix that. It also adds unit tests which will prevent regression in the future.

How Has This Been Tested?

Manually + newly created unit tests.

Screenshots (jpeg or gifs if applicable):

Before

After

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

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

[codecov]

Codecov Report

Merging #3648 into master will increase coverage by 0.24%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3648      +/-   ##
==========================================
+ Coverage   36.91%   37.15%   +0.24%     
==========================================
  Files         275      275              
  Lines        6629     6629              
  Branches     1201     1202       +1     
==========================================
+ Hits         2447     2463      +16     
+ Misses       3530     3517      -13     
+ Partials      652      649       -3

Impacted Files	Coverage Δ
editor/components/document-outline/index.js	75% <100%> (+57.14%)	⬆️

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[mtias]

I approve the change.