fix(tests): Fixes mysqli test by adding missing slowsql

[mfulb]

A EXPECT_SLOW_SQLS specification was left out of one test from PR #881 .

[newrelic-php-agent-bot]

Test Suite	Status	Result
Multiverse	cancelled 🚫	8/9 passing
SOAK	✅	56/56 passing

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.72%. Comparing base (ac8271c) to head (3ef1286).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #882   +/-   ##
=======================================
  Coverage   78.72%   78.72%           
=======================================
  Files         193      193           
  Lines       27214    27214           
=======================================
  Hits        21423    21423           
  Misses       5791     5791           

Flag	Coverage Δ
agent-for-php-7.0	77.45% <ø> (ø)
agent-for-php-7.1	77.21% <ø> (?)
agent-for-php-7.2	78.13% <ø> (ø)
agent-for-php-7.3	78.15% <ø> (ø)
agent-for-php-7.4	77.86% <ø> (ø)
agent-for-php-8.0	77.92% <ø> (ø)
agent-for-php-8.1	77.91% <ø> (ø)
agent-for-php-8.2	77.51% <ø> (ø)
agent-for-php-8.3	77.51% <ø> (ø)

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