Updated unit tests to get services/context in consistent way

All tests now using TestHelpers to create service provider and/or context and/or scoped context services. Tests that use this mechanism run isolated from each other and are quite resilient to internal refactoring. Also updated MigratorTest per issue #1050
Removed relational model and replaced with metadata model where needed.
Updated related code and tests.
Added tests to validate that issues #1077 and #1078 are fixed.
1077: Migrations: identity: false not respected when creating tables.
1078: Migrations: nullable: false not respected when creating tables.
Moved Column from Relational.Model to Migrations.Model
Moved RelationalMetadataExtensionProvider to test code. Implemented SQLiteMetadataExtensionProvider to return Relational extensions.
Added RelationalTestHelper and MigrationsTestHelper.