…eprecation on-demand (nicolas-grekas)

This PR was merged into the 2.8 branch.

Discussion
----------

[Bridge\PhpUnit] Display the stack trace of a deprecation on-demand

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This PR has been essential into making symfony#16708, and I think everyone working on a 3.0 migration will need this.

Example:

```
$ SYMFONY_DEPRECATIONS_HELPER=/FormExtensionBootstrap3LayoutTest/ ./phpunit src/Symfony/Bridge/Twig/
PHPUnit 4.8.18 by Sebastian Bergmann and contributors.

Testing src/Symfony/Bridge/Twig/
................................................
Remaining deprecation triggered by Symfony\Bridge\Twig\Tests\Extension\FormExtensionBootstrap3LayoutTest::testSingleChoiceGrouped:
The value "false" for the "choices_as_values" option is deprecated since version 2.8 and will not be supported anymore in 3.0. Set this option to "true" and flip the contents of the "choices" option instead.
Stack trace:
#0 src/Symfony/Component/Form/Extension/Core/Type/ChoiceType.php(286): trigger_error()
#1 src/Symfony/Component/OptionsResolver/OptionsResolver.php(962): Symfony\Component\Form\Extension\Core\Type\ChoiceType->Symfony\Component\Form\Extension\Core\Type\{closure}()
#2 src/Symfony/Component/Form/Extension/Core/Type/ChoiceType.php(277): Symfony\Component\OptionsResolver\OptionsResolver->offsetGet()
#3 src/Symfony/Component/OptionsResolver/OptionsResolver.php(962): Symfony\Component\Form\Extension\Core\Type\ChoiceType->Symfony\Component\Form\Extension\Core\Type\{closure}()
#4 src/Symfony/Component/OptionsResolver/OptionsResolver.php(791): Symfony\Component\OptionsResolver\OptionsResolver->offsetGet()
#5 src/Symfony/Component/Form/ResolvedFormType.php(156): Symfony\Component\OptionsResolver\OptionsResolver->resolve()
symfony#6 src/Symfony/Component/Form/FormFactory.php(119): Symfony\Component\Form\ResolvedFormType->createBuilder()
symfony#7 src/Symfony/Component/Form/FormFactory.php(48): Symfony\Component\Form\FormFactory->createNamedBuilder()
symfony#8 src/Symfony/Component/Form/Tests/AbstractBootstrap3LayoutTest.php(540): Symfony\Component\Form\FormFactory->createNamed()
symfony#9 [internal function]: Symfony\Component\Form\Tests\AbstractBootstrap3LayoutTest->testSingleChoiceGrouped()
symfony#10 {main}
KO src/Symfony/Bridge/Twig/
```

Commits
-------

5a88fb6 [Bridge\PhpUnit] Display the stack trace of a deprecation on-demand