updated FirebirdTests to fix transactions use.

It seems that the FirebirdTest project is outdated, and Transactions didn't work.
It had to do with the way connections were initialized.

Changed:
using (var db = ConnectionString.OpenDbConnection())
To:
using (var db = new OrmLiteConnectionFactory(ConnectionString,
FirebirdDialect.Provider).Open())

With this change the Firebird provider works as expected, at least regarding the use of Transactions.
Also added app.config and changed OrmLiteTestBase.GetFileConnectionString() to use it, to allow an easy connection string setup.

Only remaining problem now is at OrmLiteQueryTest.Can_select_with_filter_from_ModelWithOnlyStringFields_table().
db.ByExampleWhere() doesn't ignore NULL properties and the query returns 0 rows.

Thanks.