Wrap contents of BODY in DIV tag in tests related to Optimization Detective

[westonruter]

Summary

This PR is part of addressing #1787. It is exclusively focused on wrapping the contents of BODY in a DIV to better align with how pages are constructed by WordPress in the real world. The subsequent PR will then replace /*[1][self::HTML]/*[2][self::BODY]/*[1][self::DIV] with /HTML/BODY/DIV.

[github-actions]

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.43%. Comparing base (b059cbf) to head (62e498a).
Report is 10 commits behind head on trunk.

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #1793   +/-   ##
=======================================
  Coverage   57.43%   57.43%           
=======================================
  Files          84       84           
  Lines        6508     6508           
=======================================
  Hits         3738     3738           
  Misses       2770     2770           

Flag	Coverage Δ
multisite	57.43% <ø> (ø)
single	34.55% <ø> (ø)

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