Exception trace cannot be serialized because of closure

Problem/Motivation

Probably something has changed recently either in PHPUnit or somewhere else and we started seeing different issues related to serialization in tests. This particular issue was reported by @quietone in #3192893: [META] Serialization issues in Migration tests, which is now a meta task.

The issue is the following:

If the test is failing with exception, then PHP is trying to serialize a stack trace, which contains functions, methods, classes that have been executed. One of these functions is recognized as Closure (in other words it's anonymous function) in the stack trace (see the screenshot).

This happens because we have array_walk() inside array_walk(), so the deepest one is executed inside the closure.

PHPUnit has a related issue.

Steps to reproduce

Change a query in any source plugin and run the relevant migration test.
For example,
In d6\Term.php change ->orderBy('td.tid'); to ->orderBy('td.foo');

Then run the d6 term test.

phpunit -c core --debug -v --colors=always core/modules/taxonomy/tests/src/Kernel/Migrate/d6/MigrateTaxonomyTermTest.php

You'll see something like:

Drupal\Tests\taxonomy\Kernel\Migrate\d7\MigrateTaxonomyTermTest::testTaxonomyTerms
PHPUnit\Framework\Exception: PHP Fatal error:  Uncaught Exception: Serialization of 'Closure' is not allowed in Standard input code:80
Stack trace:
#0 Standard input code(80): serialize(Array)
#1 Standard input code(112): __phpunit_run_isolated_test()
#2 {main}
  thrown in Standard input code on line 80
Fatal error: Uncaught Exception: Serialization of 'Closure' is not allowed in Standard input code:80
Stack trace:
#0 Standard input code(80): serialize(Array)
#1 Standard input code(112): __phpunit_run_isolated_test()
#2 {main}
  thrown in Standard input code on line 80
  
/var/www/html/vendor/phpunit/phpunit/src/Util/PHP/AbstractPhpProcess.php:254
/var/www/html/vendor/phpunit/phpunit/src/Util/PHP/AbstractPhpProcess.php:171
/var/www/html/vendor/phpunit/phpunit/src/Framework/TestSuite.php:601
/var/www/html/vendor/phpunit/phpunit/src/TextUI/TestRunner.php:633
/var/www/html/vendor/phpunit/phpunit/src/TextUI/Command.php:204
/var/www/html/vendor/phpunit/phpunit/src/TextUI/Command.php:163

Proposed resolution

Add a dependency on mpyw / phpunit-patch-serializable-comparison

There is no way to add a test here because the serialization happens when the test is already executed.

Remaining tasks

Review, commit.
Think about how we can prevent this in the future.