Improve pcntl tests stability

[iamluc]

Description

It look more stable :finger_crossed:; it passed three times in a row.

There were several kinds of errors that could have occurred:

• The order of requests is not guaranteed.
• The number of requests (traces may have been grouped into a single request).
• A timeout could have occurred before the background sender finished sending traces.

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 (98839cf) to head (dc22678).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #2841   +/-   ##
=========================================
  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 98839cf...dc22678. Read the comment docs.