Simplify response mocking in unit tests

[julianlocke]

This PR makes mocking SDK responses in test a little more ergonomic. It does this in two ways:

• It provides a utility for producing a mock SDK namespace (aka a routes object) already backed by a MockTransportClient. This eliminates fiddly few lines that all tests previously needed.
• It extends the mock input interface to take, in addition to the existing JSON fixture inputs, model object (result or error objects, e.g. (Files.Metadata)) inputs.

func testExample() throws {
    let e = expectation(description: "")

    let (filesRoutes, mockTransportClient) = MockingUtilities.makeRoutesObject(forType: FilesRoutes.self)
    let webService = ExampleWebService(routes: filesRoutes)

    webService.getMetadata { result, _ in
        XCTAssertNotNil(result)
        e.fulfill()
    }

    // Mocking option 1
    let model: Files.Metadata = Files.FileMetadata(
        name: "name", id: "id", clientModified: Date(), serverModified: Date(), rev: "123456789", size: 0
    )
    try mockTransportClient.getLastRequest()?.handleMockInput(
        .success(model: model)
    )

    // Mocking option 2
    let fileMetadataJSON: [String: Any] =
    [
        ".tag": "file",
        "id": "id",
        "server_modified": "2023-12-15T13:43:32Z",
        "name": "name",
        "size": 0,
        "client_modified": "2023-12-15T13:43:32Z",
        "rev": "123456789",
        "is_downloadable": 1,
    ]
    try mockTransportClient.getLastRequest()?.handleMockInput(
        .success(json: fileMetadataJSON)
    )

    wait(for: [e])
}

This PR also guard against a stack overflow when creating a description for an object fails.

Incorporates codegen changes from: dropbox/stone#315

[jaquish]

LGTM

[jaquish]

maybe manually specify short wait time for these, instead of default

[jaquish]

add description