Introduce a new SPM-based test harness

Using the Swift Package Manager instead of xcode allows for developing
on Linux and Windows, and could potentially make life easier.

Fixes #306

Author: colinmarc

Cargo.toml

@@ -33,8 +33,6 @@ members = [
   "crates/swift-bridge-macro",
 
   "crates/swift-integration-tests",
-  "SwiftRustIntegrationTestRunner/integration-test-create-swift-package",
-  "SwiftRustIntegrationTestRunner/swift-package-rust-library-fixture",
 
   "examples/async-functions",
   "examples/codegen-visualizer",