Allow selectively enabling entity change purging in the test env (POC)

[HypeMC]

Summary 📝

This PR adds support for disabling entity change purging by default and enabling it only where needed.

Generating purge requests on every flush can noticeably slow down large test suites, even though most tests never assert on them.

A new entity_change_purging configuration option can be used to disable this behavior, for example in the test environment:

when@test:
    purgatory:
        purger: in-memory
        entity_change_purging: false

A new PHPUnit extension and #[WithEntityChangePurging] attribute can then be used to enable purging only for specific test classes or methods:

#[WithEntityChangePurging]
public function testPurgePost(): void
{
    // ...
}

When used on a test class, entity change purging is enabled for all tests in that class. When used on a test method, it is enabled only for that test. In both cases, the configured default is restored afterwards.

The new EntityChangePurgeSwitcher can also be used directly through its enable(), disable() and reset() methods, for example to temporarily disable purging while loading fixtures.

Checklist ✅

• Tests updated 🐛
• Docs updated 📚
• Changelog updated 📋
• Breaking change ⚠️

[codecov]

Codecov Report

❌ Patch coverage is 98.03922% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.57%. Comparing base (a2481de) to head (e87a4f0).

Files with missing lines	Patch %	Lines
src/PHPUnit/Metadata/AttributeReader.php	93.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                1.x     #155      +/-   ##
============================================
+ Coverage     97.48%   97.57%   +0.08%     
- Complexity      522      550      +28     
============================================
  Files            65       68       +3     
  Lines          1432     1483      +51     
============================================
+ Hits           1396     1447      +51     
  Misses           36       36              

Flag	Coverage Δ
8.1	96.35% <98.03%> (+0.12%)	⬆️
8.2	96.28% <98.03%> (+0.13%)	⬆️
8.3	96.08% <98.03%> (+0.13%)	⬆️
8.4	97.29% <98.03%> (+0.09%)	⬆️
8.5	97.29% <98.03%> (+0.09%)	⬆️

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