Changed to uppercase the SQL snippet returned from getDateAddHourExpression().
Changed to uppercase some SQL snippets.
Deleted InformixPlatform::getSubstringExpression().

The AbstractPlatform::getSubstringExpression() method works correctly.
Deleted InformixPlatform::getForUpdateSQL().

The AbstractPlatform::getForUpdateSQL() returns the correct SQL.
Added support for sequences.
Added more unit tests to InformixPlatformTest.