Minor. Use coroutine test helpers for inline/diagnostics tests

It should help with decreasing test data copy-pasting when
testing against release / pre-release coroutines

compiler/testData/codegen/box/coroutines/multiModule/inlineFunctionWithOptionalParam.kt

@@ -18,13 +18,7 @@ import COROUTINES_PACKAGE.intrinsics.*
 var result = ""
 
 fun builder(c: suspend () -> Unit) {
-    c.startCoroutine(object : Continuation<Unit> {
-        override val context = EmptyCoroutineContext
-        override fun resume(value: Unit) {
-        }
-        override fun resumeWithException(exception: Throwable) {
-        }
-    })
+    c.startCoroutine(EmptyContinuation)
 }
 
 fun box(): String {