[Behat] Fix pricing-related validation message assertions

Uncaught Symfony\Component\Debug\Exception\ContextErrorException: Warning: Cannot bind closure to scope of internal class ReflectionProperty in /Users/pamil/Projects/Sylius/Sylius/var/cache/test/appTestProjectContainer.php:28784
Stack trace:
#0 [internal function]: SyliusBundleChannelBundleDoctrineORMChannelRepository_000000004a4ae55b000000001fee2e9bc41b18016a7f6458d1a51d748b49311b->__set('_entityName', NULL)
#1 /Users/pamil/Projects/Sylius/Sylius/app/TestAppKernel.php(72): ReflectionProperty->setValue(Object(SyliusBundleChannelBundleDoctrineORMChannelRepository_000000004a4ae55b000000001fee2e9bc41b18016a7f6458d1a51d748b49311b), NULL)
#2 /Users/pamil/Projects/Sylius/Sylius/app/TestAppKernel.php(41): TestAppKernel->cleanupContainer(Object(appTestProjectContainer))
#3 /Users/pamil/Projects/Sylius/Sylius/vendor/lakion/api-test-case/src/ApiTestCase.php(85): TestAppKernel->shutdown()
#4 /Users/pamil/Projects/Sylius/Sylius/vendor/phpunit/phpunit/src/Framework/TestSuite.php(727): Lakion\ApiTestCase\ApiTestCase::ensur in /Users/pamil/Projects/Sylius/Sylius/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/HttpKernel.php on line 99

This PR was merged into the 1.7 branch.

Discussion
----------

| Q               | A
| --------------- | -----
| Branch?         | 1.7
| Bug fix?        | yes
| New feature?    | no
| BC breaks?      | no
| Deprecations?   | no
| Related tickets | 
| License         | MIT

Should resolve:
```bash
PHP Fatal error:  Uncaught Psalm\Exception\TypeParseTreeException: Unterminated bracket in /home/travis/build/Sylius/Sylius/vendor/vimeo/psalm/src/Psalm/Internal/Type/ParseTree.php:571
Stack trace:
#0 /home/travis/build/Sylius/Sylius/vendor/vimeo/psalm/src/Psalm/Type.php(171): Psalm\Internal\Type\ParseTree::createFromTokens(Array)
#1 /home/travis/build/Sylius/Sylius/vendor/vimeo/psalm/src/Psalm/Internal/Visitor/ReflectorVisitor.php(2668): Psalm\Type::parseTokens(Array)
#2 /home/travis/build/Sylius/Sylius/vendor/vimeo/psalm/src/Psalm/Internal/Visitor/ReflectorVisitor.php(2207): Psalm\Internal\Visitor\ReflectorVisitor->getAssertionParts('array<array-key...', Object(PhpParser\Node\Stmt\Function_))
#3 /home/travis/build/Sylius/Sylius/vendor/vimeo/psalm/src/Psalm/Internal/Visitor/ReflectorVisitor.php(323): Psalm\Internal\Visitor\ReflectorVisitor->registerFunctionLike(Object(PhpParser\Node\Stmt\Function_))
#4 /home/travis/build/Sylius/Sylius/vendor/nikic/php-parser/lib/PhpParser/NodeTraverser.php(200): Psalm\Internal\Visitor\Reflect in /home/travis/build/Sylius/Sylius/vendor/vimeo/psalm/src/Psalm/Internal/Type/ParseTree.php on line 571
Fatal error: Uncaught Psalm\Exception\TypeParseTreeException: Unterminated bracket in /home/travis/build/Sylius/Sylius/vendor/vimeo/psalm/src/Psalm/Internal/Type/ParseTree.php:571
Stack trace:
#0 /home/travis/build/Sylius/Sylius/vendor/vimeo/psalm/src/Psalm/Type.php(171): Psalm\Internal\Type\ParseTree::createFromTokens(Array)
#1 /home/travis/build/Sylius/Sylius/vendor/vimeo/psalm/src/Psalm/Internal/Visitor/ReflectorVisitor.php(2668): Psalm\Type::parseTokens(Array)
#2 /home/travis/build/Sylius/Sylius/vendor/vimeo/psalm/src/Psalm/Internal/Visitor/ReflectorVisitor.php(2207): Psalm\Internal\Visitor\ReflectorVisitor->getAssertionParts('array<array-key...', Object(PhpParser\Node\Stmt\Function_))
#3 /home/travis/build/Sylius/Sylius/vendor/vimeo/psalm/src/Psalm/Internal/Visitor/ReflectorVisitor.php(323): Psalm\Internal\Visitor\ReflectorVisitor->registerFunctionLike(Object(PhpParser\Node\Stmt\Function_))
#4 /home/travis/build/Sylius/Sylius/vendor/nikic/php-parser/lib/PhpParser/NodeTraverser.php(200): Psalm\Internal\Visitor\Reflect in /home/travis/build/Sylius/Sylius/vendor/vimeo/psalm/src/Psalm/Internal/Type/ParseTree.php on line 571
```

Ref. https://travis-ci.org/github/Sylius/Sylius/jobs/682877325
Ref. amphp/amp#314

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Commits
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f6819a2 [Maintenance] Upgrade psalm version