updated FirebirdTests to fix transactions use.

It seems that the FirebirdTest project is outdated, and Transaction
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.