Merge boxWithStdlib testData into box, delete BoxWithStdlib test

compiler/testData/codegen/box/controlStructures/continueInExpr.kt

@@ -0,0 +1,16 @@
+// WITH_RUNTIME
+
+fun concatNonNulls(strings: List<String?>): String {
+    var result = ""
+    for (str in strings) {
+        result += str?:continue
+    }
+    return result
+}
+
+fun box(): String {
+    val test = concatNonNulls(listOf("abc", null, null, "", null, "def"))
+    if (test != "abcdef") return "Failed: test=$test"
+
+    return "OK"
+}