Don't flag whitespace around <br/> tags

[pascalpp]

Ref: #2988

This PR simply removes BR from the set of inline elements, because I don't think there's any case where whitespace around a br tag would result in a different result rendered in the browser.

Adds example tests for code that should not be flagged by this rule:

<App>
  A
  <br/>
  B
</App>

<App>
  A<br/>
  B
</App>

<App>
  A<br/>B
</App>

<App>A<br/>B</App>

[codecov-commenter]

Codecov Report

Merging #2989 (9a140a5) into master (9aa539d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2989   +/-   ##
=======================================
  Coverage   97.59%   97.59%           
=======================================
  Files         110      110           
  Lines        7268     7268           
  Branches     2651     2651           
=======================================
  Hits         7093     7093           
  Misses        175      175           

Impacted Files	Coverage Δ
lib/rules/jsx-child-element-spacing.js	100.00% <ø> (ø)

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