Add integrations tests

[sivaprasadreddy]

The plugin can be tested using https://github.com/khmarbaise/maven-it-extension.
Implement tests to verify the successful code generation with all the supporting databases.

[romchellis]

Hi @sivaprasadreddy
I've discovered suggested library and found it quite complex for usage.
Also it doesn't work actually, it always brings JOOQ compiler errors due to the discrepancy between the real structure of the project and the emulation.
Please take a look on my Pull Request, i achieved it much more simpler way using maven-plugin-testing-harness.
If you like the approach, i will add more tests further