Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated

compiler/testData/codegen/box/bridges/simpleUpperBound.kt

@@ -0,0 +1,17 @@
+open class A<T : Number> {
+    open fun foo(t: T) = "A"
+}
+
+class Z : A<Int>() {
+    override fun foo(t: Int) = "Z"
+}
+
+
+fun box(): String {
+    val z = Z()
+    return when {
+        z.foo(0)            != "Z" -> "Fail #1"
+        (z : A<Int>).foo(0) != "Z" -> "Fail #2"
+        else -> "OK"
+    }
+}