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

@@ -19,6 +19,8 @@ import SourceKitLSP
 import SwiftExtensions
 import TSCBasic
 import ToolchainRegistry
+import Basics
+import PackageModel
 
 package struct IndexedSingleSwiftFileTestProject {
   enum Error: Swift.Error {
@@ -79,17 +81,23 @@ 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 = try? WindowsPlatformInfo(
+        reading: AbsolutePath(validating: platform.appendingPathComponent("Info.plist").path),
+        observabilityScope: nil,
+        filesystem: Basics.localFileSystem
+      ) {
+        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