Merge boxMultiFile testData into box, delete BoxMultiFile test

compiler/testData/codegen/box/regressions/kt5445.kt

@@ -0,0 +1,27 @@
+// WITH_RUNTIME
+// FILE: 1.kt
+
+package test2
+
+import test.A
+
+public fun box(): String {
+    return B().test(B())
+}
+
+public class B : A() {
+    public fun test(other:Any): String {
+        if (other is B && other.s == 2) {
+            return "OK"
+        }
+        return "fail"
+    }
+}
+
+// FILE: 2.kt
+
+package test
+
+open class A {
+    @JvmField protected val s = 2;
+}