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.

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/Public/XCTestCase.swift

@@ -54,7 +54,23 @@ open class XCTestCase: XCTest {
  return 1
  }
 
- internal static let subsystemQueue = DispatchQueue(label: "org.swift.XCTestCase")
+ #if DISABLE_XCTWAITER && os(WASI)
+ /// 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