moved mysql specific test into pdo-mysql

[staabm]

pgsql does not like these tests

1) staabm\PHPStanDba\Tests\DbaInferenceTest::testFileAsserts with data set "/home/runner/work/phpstan-dba/phpstan-dba/tests/default/data/pdo.php:158" ('type', '/home/runner/work/phpstan-dba...do.php', PHPStan\Type\Constant\ConstantStringType Object (...), PHPStan\Type\ObjectType Object (...), 158)
Expected type PDOStatement<array{adaid: int<-32768, 32767>}>, got type PDOStatement in /home/runner/work/phpstan-dba/phpstan-dba/tests/default/data/pdo.php on line 158.
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-'PDOStatement<array{adaid: int<-32768, 32767>}>'
+'PDOStatement'

phar:///home/runner/work/phpstan-dba/phpstan-dba/vendor/phpstan/phpstan/phpstan.phar/src/Testing/TypeInferenceTestCase.php:56
/home/runner/work/phpstan-dba/phpstan-dba/tests/default/DbaInferenceTest.php:77

2) staabm\PHPStanDba\Tests\DbaInferenceTest::testFileAsserts with data set "/home/runner/work/phpstan-dba/phpstan-dba/tests/default/data/pdo-prepare.php:66" ('type', '/home/runner/work/phpstan-dba...re.php', PHPStan\Type\Constant\ConstantStringType Object (...), PHPStan\Type\ObjectType Object (...), 66)
Expected type PDOStatement<array{adaid: int<-32768, 32767>, 0: int<-32768, 32767>}>, got type PDOStatement in /home/runner/work/phpstan-dba/phpstan-dba/tests/default/data/pdo-prepare.php on line 66.
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-'PDOStatement<array{adaid: int<-32768, 32767>, 0: int<-32768, 32767>}>'
+'PDOStatement'

since these are rather edge-cases we consider them supported only on mysql for now