Shorten package manifest declarations in tests

[ahoppen]

All package manifest declarations for SwiftPMTestProject start with swift-tools-version and import PackageDescription. I think the tests could be simplified SwiftPMTestProject would automatically add those two lines if they don’t exist in the manifest declaration. That way you can specify them manually but don‘t have to.

rdar://126484621

[ahoppen]

Implemented in #1391