Split out IntegrationTests into separate testTarget

[fabianfett]

This PR splits out all the integration tests into a separate test target. This makes a number of things easier in day to day development:

1. Unit tests can be run without setting up a test database.
2. We can observe the test coverage of our Unit tests only.
3. This allows us to decouple testing the language vs. testing the functionality against a database.

I think after this change we could change CI to:

Unit testing for:

      matrix:
        swiftver:
          - swift:5.2
          - swift:5.3
          - swift:5.4
          - swiftlang:swift/5.5-nightly
          - swiftlang:swift/main-nightly
        swiftos:
          - focal

Integration Testing tests for:

      matrix:
        dbimage:
          - postgres:13
          - postgres:12
          - postgres:11
        dbauth:
          - trust
          - md5
          - scram-sha-256
        swiftver:
          - swift:5.4
        swiftos:
          - focal

[glbrntt]

Seems like a very good idea to me. Looks good aside from two nits.

[glbrntt]

nit: might as well pull this out into a separate function given it's used below as well

[fabianfett]

This is previous code just copy and pasted here. If I try to refactor this we end up with a warning. Would love to keep as is for now. We should refactor the perf tests to allocation tests anyway.

[glbrntt]

Fine by me to leave this for now. Out of curiosity, why do we end up with a warning?

[fabianfett]

"This code is never reached" for debug builds.

[0xTim]

This LGTM but you haven't changed any of the CI setup - are you planning to?