Provide simple PHPUnit constraint

[Vinai]

This is a simple PHPUnit integration for php-quickcheck.
It consists of a custom assertion and a static factory method.

To use this in a PHPUnit test, call

$this->assertThat(Gen::forAll($generators, $prop), Prop::check(100));

The output of a failing test looks like:

Failed asserting that property is true.
Tests runs: 16, failing size: 15, seed: 1578486446175, smallest shrunk value(s):
array (
  0 => <failed shrunk value>,
)

The smallest shrunk value is displayed using var_export so it can be copied and pasted into the property Closure for easy debugging.

Please consider the constraint class name and the factory method name as a suggestion - happy to rename them to something better.

Due to the method signatures of the overridden methods from PHPUnit\Framework\Constraint\Constraint, the constraint class is only PHP 7 compatible.
It would be possible to provide alternative implementations of the custom assertion for ancient PHPUnit versions that are compatible with PHP 5.6, but I doubt it is worth the effort.

[Vinai]

This PR is a more lightweight alternative to #7. I would like to think it behaves a bit like the defspec macro from clojure.test.check.
It should also work well in conjunction with #7 actually.

[Vinai]

Sorry about all the updates, sitting in a train trying to find better names.

[steos]

This seems great, thank you very much!
Let me know if I can merge it or if you still want to make some changes here.

[Vinai]

I'm good with the current shape.

[Vinai]

Well, I can add a blurb to the readme maybe.

[Vinai]

Now I think I'm okay with the PR, so if you are happy with it, too, it's ready to be merged.

[Vinai]

Since the README can't be the cause, maybe some changes you made while adding dontShrink broke the PHP 5 builds?

[steos]

It's all good. Looks like there was some temporary error with composer install and the random shrink test sometimes doesn't generate a failing case.

Thanks again for your efforts!