1. Got rid of hard-coded exception mocks in favor of PHPUnit-generated ones where possible.
2. Reworked AbstractDriverTest::testConvertsException() to use a data provider to make testing all types of exceptions independent.
3. Updated PHPUnit since the reworked code relies on sebastianbergmann/phpunit#3604.

As per @belgattitude's review.

When following error message is shown, then it will get converted to the
correct Exception classs of NotNullConstraintViolationException:

  PDOException: SQLSTATE[HY000]: General error: 1364 Field '...' doesn't have a
  default value