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/methodFromTrait.kt

@@ -0,0 +1,16 @@
+trait A<T, U> {
+    fun foo(t: T, u: U) = "A"
+}
+
+class Z<T> : A<T, Int> {
+    override fun foo(t: T, u: Int) = "Z"
+}
+
+fun box(): String {
+    val z = Z<Int>()
+    return when {
+        z.foo(0, 0)                 != "Z" -> "Fail #1"
+        (z : A<Int, Int>).foo(0, 0) != "Z" -> "Fail #2"
+        else -> "OK"
+    }
+}