Concurrency: Add startOnMainActor test

Adding a test to ensure that we're running things while the main actor
has other things enqueued.

Author: etcwilde

test/Concurrency/Runtime/startOnMainActor.swift

@@ -0,0 +1,88 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -Xfrontend -disable-availability-checking %s -o %t/a.out
+// RUN: %target-codesign %t/a.out
+// RUN:  %target-run %t/a.out
+
+// REQUIRES: executable_test
+// REQUIRES: concurrency
+// REQUIRES: concurrency_runtime
+// UNSUPPORTED: back_deployment_runtime
+// UNSUPPORTED: back_deploy_concurrency
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: freestanding
+
+import StdlibUnittest
+@_spi(MainActorUtilities) import _Concurrency
+
+func doStuffAsync() async {
+    await Task.sleep(500)
+}
+
+let tests = TestSuite("StartOnMainActor")
+
+tests.test("startOnMainActor") {
+    // "global" variables for this test
+    struct Globals {
+        @MainActor
+        static var ran = false
+    }
+
+    @MainActor
+    func run() async {
+        Globals.ran = true
+        await doStuffAsync()
+    }
+
+    // enqueue item on the MainActor
+    let t1 = Task { @MainActor in
+        await Task.sleep(1000)
+    }
+
+    expectFalse(Globals.ran)
+
+    // Run something with side-effects on the main actor
+    let t2 = Task.startOnMainActor {
+        return await run()
+    }
+
+    expectTrue(Globals.ran)
+    await t1.value
+    await t2.value
+}
+
+tests.test("throwing startOnMainActor") {
+    // "global" variables for this test
+    struct Globals {
+        @MainActor
+        static var ran = false
+    }
+
+    struct StringError: Error {
+        let message: String
+    }
+
+    @MainActor
+    func run() async throws {
+        Globals.ran = true
+        await doStuffAsync()
+        throw StringError(message: "kablamo!")
+    }
+
+    // enqueue item on the MainActor
+    let t1 = Task { @MainActor in
+        await Task.sleep(1000)
+    }
+
+    expectFalse(Globals.ran)
+
+    // Run something with side-effects on the main actor
+    let t2 = Task.startOnMainActor {
+        return try await run()
+    }
+
+    expectTrue(Globals.ran)
+    await t1.value
+    expectNil(try? await t2.value)
+}
+
+await runAllTestsAsync()