Fix Redis tests flakiness

[iamluc]

Description

Fix this error

1) DDTrace\Tests\Integrations\PHPRedis\V4\PHPRedisClusterTest::testMethodsOnlySpan with data set "bgRewriteAOF" ('bgRewriteAOF', null)
RedisClusterException: Unable to send command at a specific node

Some Redis commands can fail, so we must account for this in the assertion.

So far, I've only encountered the issue with the PHPRedisClusterTest. Let's wait and see if we also need it for the regular Redis client.

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.06%. Comparing base (ebaa1ca) to head (6759fad).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #2848   +/-   ##
=========================================
  Coverage     81.06%   81.06%           
  Complexity     2516     2516           
=========================================
  Files           146      146           
  Lines         14653    14653           
=========================================
  Hits          11879    11879           
  Misses         2774     2774           

Flag	Coverage Δ
tracer-extension	78.18% <ø> (ø)
tracer-php	82.27% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ebaa1ca...6759fad. Read the comment docs.