Remove DispatchShims.swift and replace it with a conditional typealias

The use of DispatchQueue in XCTest is now very limited, and it's only used
in a single place in XCTestCase.swift.

Author: kateinoigakukun

CMakeLists.txt

@@ -38,7 +38,6 @@ add_library(XCTest
   Sources/XCTest/Private/WaiterManager.swift
   Sources/XCTest/Private/IgnoredErrors.swift
   Sources/XCTest/Private/XCTestCase.TearDownBlocksState.swift
-  Sources/XCTest/Private/DispatchShims.swift
   Sources/XCTest/Public/XCTestRun.swift
   Sources/XCTest/Public/XCTestMain.swift
   Sources/XCTest/Public/XCTestCase.swift

Sources/XCTest/Private/DispatchShims.swift

@@ -54,7 +54,23 @@ open class XCTestCase: XCTest {
         return 1
     }
 
-    internal static let subsystemQueue = DispatchQueue(label: "org.swift.XCTestCase")
+    #if DISABLE_XCTWAITER
+    /// Single-threaded queue without any actual queueing
+    struct SubsystemQueue {
+        init(label: String) {}
+
+        func sync<T>(_ body: () -> T) -> T {
+            body()
+        }
+        func async(_ body: @escaping () -> Void) {
+            body()
+        }
+    }
+    #else
+    typealias SubsystemQueue = DispatchQueue
+    #endif
+
+    internal static let subsystemQueue = SubsystemQueue(label: "org.swift.XCTestCase")
 
     #if !DISABLE_XCTWAITER
     @MainActor