SKTestSupport: remove hardcoded version for XCTest

The XCTest and Testing version information is encoded into the
`PlatformInfo.plist` that is at the root of the platform. Use this to
determine the path for XCTest. This allows us to migrate the XCTest
location into an appropriate versioned directory.

Author: compnerd

Sources/SKTestSupport/IndexedSingleSwiftFileTestProject.swift

@@ -20,6 +20,49 @@ import SwiftExtensions
 import TSCBasic
 import ToolchainRegistry
 
+package struct WindowsPlatformInfo {
+    package struct DefaultProperties {
+        /// XCTEST_VERSION
+        /// specifies the version string of the bundled XCTest.
+        public let xctestVersion: String
+
+        /// SWIFT_TESTING_VERSION
+        /// specifies the version string of the bundled swift-testing.
+        public let swiftTestingVersion: String?
+
+        /// SWIFTC_FLAGS
+        /// Specifies extra flags to pass to swiftc from Swift Package Manager.
+        public let extraSwiftCFlags: [String]?
+    }
+
+    public let defaults: DefaultProperties
+}
+
+extension WindowsPlatformInfo.DefaultProperties: Decodable {
+    enum CodingKeys: String, CodingKey {
+    case xctestVersion = "XCTEST_VERSION"
+    case swiftTestingVersion = "SWIFT_TESTING_VERSION"
+    case extraSwiftCFlags = "SWIFTC_FLAGS"
+    }
+}
+
+extension WindowsPlatformInfo: Decodable {
+    enum CodingKeys: String, CodingKey {
+    case defaults = "DefaultProperties"
+    }
+}
+
+extension WindowsPlatformInfo {
+    package init?(reading path: URL) {
+        do {
+            let data: Data = try Data(contentsOf: path)
+            self = try PropertyListDecoder().decode(WindowsPlatformInfo.self, from: data)
+        } catch {
+            return nil
+        }
+    }
+}
+
 package struct IndexedSingleSwiftFileTestProject {
   enum Error: Swift.Error {
     case swiftcNotFound
@@ -79,17 +122,19 @@ package struct IndexedSingleSwiftFileTestProject {
       // The following are needed so we can import XCTest
       let sdkUrl = URL(fileURLWithPath: sdk)
       #if os(Windows)
-      let xctestModuleDir =
-        sdkUrl
-        .deletingLastPathComponent()
-        .deletingLastPathComponent()
-        .appendingPathComponent("Library")
-        .appendingPathComponent("XCTest-development")
-        .appendingPathComponent("usr")
-        .appendingPathComponent("lib")
-        .appendingPathComponent("swift")
-        .appendingPathComponent("windows")
-      compilerArguments += ["-I", try xctestModuleDir.filePath]
+      let platform = sdkUrl.deletingLastPathComponent().deletingLastPathComponent().deletingLastPathComponent()
+      if let info = WindowsPlatformInfo(reading: platform.appendingPathComponent("Info.plist")) {
+        let xctestModuleDir =
+          platform
+          .appendingPathComponent("Developer")
+          .appendingPathComponent("Library")
+          .appendingPathComponent("XCTest-\(info.defaults.xctestVersion)")
+          .appendingPathComponent("usr")
+          .appendingPathComponent("lib")
+          .appendingPathComponent("swift")
+          .appendingPathComponent("windows")
+        compilerArguments += ["-I", try xctestModuleDir.filePath]
+      }
       #else
       let usrLibDir =
         sdkUrl