Do not generate a return statement in the constructor of a test double

[lazerg]

When an interface (or class) that declares a __construct() method is doubled, the generated constructor ends with return $__phpunit_result;. A constructor cannot declare a return type, so ReflectionMapper resolves it to UnknownType (neither void nor never) and DoubledMethod::generateCode() emits the return statement. On PHP 8.6 this triggers the Returning a value from a constructor is deprecated deprecation.

This suppresses the return statement when the generated method is a constructor. The existing generator test for #4139 (which doubles an interface declaring a constructor) is updated to the corrected output; it fails before this change and passes after.

Closes #6797

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.00%. Comparing base (7c4ce79) to head (4c4dc4c).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##               12.5    #6800   +/-   ##
=========================================
  Coverage     96.00%   96.00%           
- Complexity     7568     7569    +1     
=========================================
  Files           798      798           
  Lines         23180    23180           
=========================================
  Hits          22253    22253           
  Misses          927      927           

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[sebastianbergmann]

Thank you for your contribution and for taking the time to prepare this pull request. I appreciate the effort.

However, I've decided not to merge it as-is, since this issue needs to be addressed across the 8.5, 9.6, 10.5, 11.5, 12.5, 13.2, and main branches. I'll take care of this myself next week, in one pass; starting with a fix for the oldest supported branch and then merging/adapting it forward through the newer branches.