[Tests] Suspend Codegen Tests Improvements

This commit enables the execution of suspend box tests in a separate
test. It's a QoL improvement in the existing bb tests but, motivating
these changes, enables the new debugger stepping tests to step
coroutine code.

compiler/testData/debug/localVariables/suspend/simple.kt

@@ -0,0 +1,10 @@
+// WITH_COROUTINES
+// FILE: test.kt
+
+suspend fun box() {
+    var x = 1
+}
+
+// LOCAL VARIABLES
+// TestKt:5: $completion:kotlin.coroutines.Continuation=helpers.ResultContinuation
+// TestKt:6: $completion:kotlin.coroutines.Continuation=helpers.ResultContinuation, x:int=1:int