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/boxInline/suspend/tryCatchStackTransform.kt

@@ -1,6 +1,7 @@
 // FILE: inlined.kt
 // COMMON_COROUTINES_TEST
 // WITH_RUNTIME
+// WITH_COROUTINES
 // NO_CHECK_LAMBDA_INLINING
 
 suspend inline fun inlineMe(c: suspend (String) -> String, d: suspend () -> String): String {
@@ -19,19 +20,10 @@ suspend inline fun crossinlineMe(crossinline c: suspend (String) -> String, cros
 // COMMON_COROUTINES_TEST
 
 import COROUTINES_PACKAGE.*
+import helpers.*
 
 fun builder(c: suspend () -> Unit) {
-    c.startCoroutine(object: Continuation<Unit> {
-        override val context: CoroutineContext
-            get() = EmptyCoroutineContext
-
-        override fun resume(value: Unit) {
-        }
-
-        override fun resumeWithException(exception: Throwable) {
-            throw exception
-        }
-    })
+    c.startCoroutine(EmptyContinuation)
 }
 
 suspend fun yieldString(s: String) = s