Ensure project migrations are executed when running specs

[ellmetha]

Description

Right now, when the specs of a Marten project are executed, the models of the project will be created at the database level once. Later on, if the same database is reused for running specs the same tables will be reused if they already exist. Model tables are created directly without running project migrations ; by default we should probably ensure that project migrations are used unless configured otherwise.

Let's introduce the ability to run project migrations when setuping spec databases while giving the ability to disable this behaviour through the use of a dedicated configuration option.