Merge boxWithStdlib testData into box, delete BoxWithStdlib test

compiler/testData/codegen/box/strings/forInString.kt

@@ -0,0 +1,13 @@
+// WITH_RUNTIME
+
+fun foo(): Int {
+    var sum = 0
+    for (c in "239")
+        sum += (c.toInt() - '0'.toInt())
+    return sum
+}
+
+fun box(): String {
+    val f = foo()
+    return if (f == 14) "OK" else "Fail $f"
+}

compiler/testData/codegen/box/strings/stringPlusOnlyWorksOnString.kt

@@ -0,0 +1,9 @@
+// WITH_RUNTIME
+
+import java.util.ArrayList
+
+fun box(): String {
+    var x: MutableCollection<Int> = ArrayList()
+    x + ArrayList()
+    return "OK"
+}