add new test that now passes because of patch by @phausler

See previous commit in this PR.
Test is based on one from rdar://74281361

Author: kavon

test/Concurrency/Runtime/executor_deinit3.swift

@@ -0,0 +1,76 @@
+// RUN: %target-run-simple-swift(-parse-as-library -Xfrontend -enable-experimental-concurrency %import-libdispatch) | %FileCheck %s
+
+// REQUIRES: executable_test
+// REQUIRES: concurrency
+// REQUIRES: libdispatch
+
+// for sleep
+#if canImport(Darwin)
+    import Darwin
+#elseif canImport(Glibc)
+    import Glibc
+#endif
+
+class Runner {
+    func run() async {
+        while !Task.isCancelled {
+            sleep(1)
+        }
+    }
+}
+
+actor Container {
+    var generation = 0
+    var runners = [Int : Task.Handle<Void, Never>]()
+
+    func build(_ n: Int) {
+        for _ in 0..<n {
+            let id = generation
+            generation += 1
+            let t = Task.runDetached { [weak self] in
+                let r = Runner()
+                await r.run()
+                await self?.remove(id)
+            }
+            runners[id] = t
+        }
+    }
+
+    func cancelAll() {
+        var count = 0
+        for (_, v) in runners {
+            v.cancel()
+            count += 1
+        }
+        print("Cancelled \(count) runners.")
+    }
+
+    deinit {
+        print("deinit Container with \(runners.count) runners")
+    }
+
+    func remove(_ id: Int) {
+        runners.removeValue(forKey: id)
+    }
+}
+
+// CHECK: starting
+// CHECK: Cancelled 5 runners.
+
+// FIXME: this doesn't work until we have https://github.com/apple/swift/pull/36298
+// COM: deinit Container with {{[0-9]+}} runners
+
+@main struct RunIt {
+    static func startTest() async {
+        let c = Container()
+        await c.build(5)
+        sleep(5)
+        await c.cancelAll()
+    }
+
+static func main() async {
+        print("starting")
+        await RunIt.startTest()
+        sleep(5)
+    }
+}