Cleanup tests with new snapshot method

[liamnichols]

• related Align behaviour of output directory and make --clean a little safer #88

Prior to now, the tests would generate and assert snapshots in a relatively verbose way which gets tricky around file paths. This would make my job harder when adjusting behaviour on #88 so instead I decided to clean things up a little.

In this change, I introduce a new snapshot(spec:name:arguments:congifuration:) method and called it in every single test method.

func testEdgecasesRename() throws {
    try snapshot(
        spec: .edgecases,
        name: "edgecases-rename",
        arguments: [
            "--package", "edgecases-rename"
        ],
        configuration: """
        {
            "rename": {
                "properties": {
                    "ContainerA.Child.Child.renameMe": "onlyItRenamed"
                },
                "entities": {
                    "ApiResponse": "APIResponse",
                    "Status": "State"
                }
            }
        }
        """
    )
}

This method then internally handles the --output path and adds a temporary workaround to account for the inconsistencies between --package and --module.

Additionally, this method works to separate the concept of a 'name' and 'package'. Currently, every generated snapshot has a different package name because package name == directory name.

Moving forward, we'll also break that concept so that name == test name and package name == whatever you provide (it doesn't really matter). By doing this, we can use #function and it'll make things simpler. I'll do that in a separate change though.

I also renamed GenerateBaseTests to GenerateTestCase to better distinguish that it's not tests but in fact an XCTestCase subclass/base class to be used by Generate related tests.