Marked some test classes abstract

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.

tests/Doctrine/Tests/DBAL/Functional/Schema/SchemaManagerFunctionalTestCase.php

@@ -46,7 +46,7 @@ use function strlen;
 use function strtolower;
 use function substr;
 
-class SchemaManagerFunctionalTestCase extends DbalFunctionalTestCase
+abstract class SchemaManagerFunctionalTestCase extends DbalFunctionalTestCase
 {
     /** @var AbstractSchemaManager */
     protected $schemaManager;

tests/Doctrine/Tests/DbalFunctionalTestCase.php

@@ -18,7 +18,7 @@ use function is_scalar;
 use function strpos;
 use function var_export;
 
-class DbalFunctionalTestCase extends DbalTestCase
+abstract class DbalFunctionalTestCase extends DbalTestCase
 {
     /**
      * Shared connection when a TestCase is run alone (outside of it's functional suite)

tests/Doctrine/Tests/DbalPerformanceTestCase.php

@@ -11,7 +11,7 @@ use function microtime;
  * and stopTiming at the end of all tests. Tests that do not start or stop
  * timing will fail.
  */
-class DbalPerformanceTestCase extends DbalFunctionalTestCase
+abstract class DbalPerformanceTestCase extends DbalFunctionalTestCase
 {
     /**
      * time the test started