Checking that the platform supports foreign key is not the right case
here, because we should check if we can create them by themselves, after
the table is created. It is not the case for Sqlite.

Closes #3990

Upstream libs and tools have improvements that make them unnecessary.

also simplifies the code and now exactly matches postgresql search rules

When the connection is lost or is closed, subsequent transaction will no longer be nested because they started in a brand new session. Our internal representation of the nesting shold take this into account

Internally, ArrayStatement expects every row to be represented as an array regardless of the fetch mode, however FetchMode::COLUMN produces one value per row.

The method is not used for anything else than skipping tests for specific drivers. Cross-driver portability should be established by drivers, not outside of them based on their name.

on linux running as root.

1. SchemaManagerFunctionalTestCase can be run only via an extending class. Otherwise, it skips all tests. If the test case is represented as an abstract class, PhpStorm will offer a choise of concrete classes which is very handy when debugging test failures.
2. DbalFunctionalTestCase and DbalPerformanceTestCase are meant to be used as base classes and don't contain any tests.