Introduce tests that cover behaviour explained in #4457 (#4509)

Author: qwwdfsad

kotlinx-coroutines-core/common/test/ScopedBuildersCancelledStartTest.kt

@@ -0,0 +1,57 @@
+package kotlinx.coroutines
+
+import kotlinx.coroutines.testing.TestBase
+import kotlin.coroutines.EmptyCoroutineContext
+import kotlin.test.Test
+import kotlin.test.assertTrue
+
+// See https://github.com/Kotlin/kotlinx.coroutines/issues/4457
+class ScopedBuildersCancelledStartTest : TestBase() {
+
+    @Test
+    fun testCancelledCoroutineScope() = runTest(expected = { it is CancellationException }) {
+        cancel()
+        coroutineScope {
+            finish(1)
+        }
+        expectUnreached()
+    }
+
+    @Test
+    fun testCancelledSupervisorScope() = runTest(expected = { it is CancellationException }) {
+        cancel()
+        supervisorScope {
+            finish(1)
+        }
+        expectUnreached()
+    }
+
+    @Test
+    fun testCancelledWithTimeout() = runTest(expected = { it is CancellationException }) {
+        cancel()
+        withTimeout(Long.MAX_VALUE) {
+            finish(1)
+        }
+        expectUnreached()
+    }
+
+    @Test
+    fun testWithContext() = runTest(expected = { it is CancellationException }) {
+        cancel()
+        withContext(EmptyCoroutineContext) {
+            assertTrue(coroutineContext.job.isCancelled)
+            expect(1)
+        }
+        finish(2)
+    }
+
+    @Test
+    fun testWithContextWithDispatcher() = runTest(expected = { it is CancellationException }) {
+        cancel()
+        withContext(Dispatchers.Unconfined) {
+            assertTrue(coroutineContext.job.isCancelled)
+            expect(1)
+        }
+        finish(2)
+    }
+}