[ticket/13733] Allow non-migration files inside migrations folder
* marc1706/ticket/13733:
[ticket/13733] Remove validate_classes method argument
[ticket/13733] Make sure migration classes always have same order in tests
[ticket/13733] Properly test setting validate_classes to false/true
[ticket/13733] Use interface for migratinos
[ticket/13733] Update comment explaining migration class validation
[ticket/13733] Add isInstantiable() check.
[ticket/13733] Braces on their own lines
[ticket/13733] Allow tests the skip class validation
[ticket/13733] Properly handle nonexistent classes as well
[ticket/13733] Handle nonexistent classes as well
[ticket/13733] Only use migration classes that extension the base migration class.
b7db2717b2