Add closure mocks

[mnapoli]

The problem

Mocking closures is doable today by mocking __invoke on an invokable class. However, this is troublesome, as it requires writing an invokable class and mocking a class method (which is confusing, hard to figure out, and makes tests harder to understand afterward).

Here is another approach that is verbose and fragile:

    public function test_validation(): void
    {
        $rule = new CustomLaravelValidationRule();
        
        $failed = false;
        $rule->validate('code', 'abcd', function () use (&$failed) {
            return $failed = true;
        });
        $this->assertTrue($failed);
    }

The solution

This new helper makes things easier via a new $this->createClosureMock() method.

$mock = $this->createClosureMock();

$mock->expectsClosure($this->once())
    ->willReturn(123);

$this->assertSame(123, $mock());

Here's the test above rewritten using the new helper:

    public function test_validation(): void
    {
        $rule = new CustomLaravelValidationRule();
        
        $mock = $this->createClosureMock();
        $mock->expectsClosure($this->never());

        $rule->validate('code', 'abcd', $mock);
    }

This is essentially just a shortcut to the workaround that is doable today, so this PR should not introduce too much new code.

Related to #3536

Note that #3536 mentioned a workaround using stdClass. I was not able to make it work: PHPUnit refused to mock stdClass's __invoke method because it does not exist.

This is, to me, yet another reason to have such a helper.

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.43%. Comparing base (a6390f9) to head (f94fa26).
Report is 525 commits behind head on main.

Files with missing lines	Patch %	Lines
.../Framework/MockObject/Runtime/Stub/ClosureMock.php	71.42%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5759      +/-   ##
============================================
- Coverage     94.43%   94.43%   -0.01%     
- Complexity     6663     6667       +4     
============================================
  Files           709      710       +1     
  Lines         20104    20113       +9     
============================================
+ Hits          18986    18993       +7     
- Misses         1118     1120       +2     

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[mnapoli]

I rebased and the static analysis error was solved 🎉

This is ready to be reviewed/merged.

The code coverage is not happy that __invoke() is not covered, but this is an empty method and it's the one being mocked, so that makes sense, I think it's perfectly fine that it's not covered. If you want me to find a way to force coverage let me know.