Refactor Integration Tests to separate DBMS tests from driver tests

[Ste1io]

Currently there is no distinction between DBMS tests and supported drivers/libraries. This creates both ambiguity, and results in duplicate code.

Adding an additional inheritance level for driver tests, which inherit from DBMS tests, would remove the duplicate test code that's DBMS specific, and encapsulate driver-specific tests moving forward.

[Ste1io]

The least-intrusive refactor requiring minimal structural changes (afaik) is below. An alternative approach would be to move "DriverTests" out into a sibling directory that mirrors "DbmsTests", and move the base tests out next to both of them. Both approaches would still use the same inheritance structure, namely Drivers -> Dbms -> Base.

Dbms tests would need to override both constructors that BaseTests offers; otherwise, the inheritance should work fine as-is.

Feedback certainly welcome.

PetaPoco\PetaPoco.Tests.Integration\.
│   AppSetting.cs
│   appsettings.json
│   PetaPoco.Tests.Integration.csproj
│
├───Databases
│   │   # Base test classes
│   │   DBTestProvider.cs
│   │   BaseDatabase.cs
│   │   BaseDatabaseTests.cs
│   │   BaseExecuteTests.cs
│   │   BaseQueryTests.cs
│   │   ...
│   │   
│   ├───MSSql
│   │   │   # MS SQL DBMS tests
│   │   │   MSSqlDbmsDatabaseTests.cs
│   │   │   MSSqlDbmsDBTestProvider.cs
│   │   │   MSSqlDbmsExecuteTests.cs
│   │   │   MSSqlDbmsQueryTests.cs
│   │   │   ...
│   │   │
│   │   ├───SystemData
│   │   │       # MS SQL driver tests for legacy "System.Data.SqlClient"
│   │   │       MSSqlDatabaseTests.cs
│   │   │       MSSqlTestProvider.cs
│   │   │       MSSqlExecuteTests.cs
│   │   │       MSSqlQueryTests.cs
│   │   │       ...
│   │   │
│   │   └───MSData
│   │           # MS SQL driver tests for modern "Microsoft.Data.SqlClient"
│   │           MSSqlMsDataDatabaseTests.cs
│   │           MSSqlMsDataDBTestProvider.cs
│   │           MSSqlMsDataExecuteTests.cs
│   │           MSSqlMsDataQueryTests.cs
│   │           ...
│   │
│   ├───Postgres
│   │   │   # Postgres Dbms tests
│   │   │   PostgresDbmsDatabaseTests.cs
│   │   │   PostgresDbmsDBTestProvider.cs
│   │   │   PostgresDbmsExecuteTests.cs
│   │   │   PostgresDbmsQueryTests.cs
│   │   │   ...
│   │   │
│   │   └───Npgsql
│   │           # Postgres driver tests for "Npgsql"
│   │           PostgresNpgsqlDatabaseTests.cs
│   │           PostgresNpgsqlDBTestProvider.cs
│   │           PostgresNpgsqlExecuteTests.cs
│   │           PostgresNpgsqlQueryTests.cs
│   │           ...
│   │           
│   └───...
│
├───Models
│   │   BugInvestigations.cs
│   │   Note.cs
│   │   Order.cs
│   │   Person.cs
│   │   ...
│   │
│   └───MSSql
│           StorePerson.cs
│
└───Scripts
        MSSQLBuildDatabase.sql
        PostgresBuildDatabase.sql
        ...

[Ste1io]

Should have a PR ready to open this weekend. Any issues with using child classes for the drivers @pleb @asherber?