Added regression test

[staabm]

closes phpstan/phpstan#11447

was fixed with #3871

before this fix

1) PHPStan\Rules\Arrays\NonexistentOffsetInArrayDimFetchRuleTest::testBug11447
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-'
+'06: Offset 'key1' might not exist on array<mixed>.
 '

/Users/m.staab/Documents/GitHub/phpstan-src/src/Testing/RuleTestCase.php:165
/Users/m.staab/Documents/GitHub/phpstan-src/tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php:877

[phpstan-bot]

This pull request has been marked as ready for review.

[ondrejmirtes]

Thank you!