test(Dropdown): add regression coverage for Checkbox nested in Dropdown.Item/Header

[AmanUllah687]

Closes #2121

What

Adds regression tests confirming that a Checkbox's onClick handler
fires correctly when nested inside a Dropdown.Item and inside a
Dropdown.Header.

Why

#2121 reports that onClick doesn't fire on a Checkbox nested inside
a Dropdown (as an item or a header). That issue is currently locked
with reason "resolved," so I wasn't able to comment there directly,
hence explaining here instead.

I couldn't reproduce the bug against current master or the published
2.1.5. I rendered a Checkbox inside both a Dropdown.Item and a
Dropdown.Header and simulated a real mousedown → mouseup → click
sequence; onClick fired correctly in both cases.

Digging through git history, Checkbox's click-handling logic was
rewritten for unrelated reasons in the fixes referenced inline as
#3351 and #3433, and that rewrite looks to have fixed this issue as a
side effect. No test ever verified it though, so this PR adds that
coverage and closes the loop on the original report.

Testing

• Added two tests under describe('Checkbox child', ...) in
  Dropdown-test.js
• yarn test passes locally (11033 tests completed, 0 failed)

[vercel]

@AmanUllah687 is attempting to deploy a commit to the Semantic UI Team on Vercel.

A member of the Team first needs to authorize it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.53%. Comparing base (e0a42b5) to head (2a23e7c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4541   +/-   ##
=======================================
  Coverage   99.53%   99.53%           
=======================================
  Files         186      186           
  Lines        3463     3463           
=======================================
  Hits         3447     3447           
  Misses         16       16           

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