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/propertyDiamond.kt

@@ -0,0 +1,18 @@
+trait A<O, K> {
+    val o: O
+    val k: K
+}
+
+trait B<K> : A<String, K>
+
+trait C<O> : A<O, String>
+
+class D : B<String>, C<String> {
+    override val o = "O"
+    override val k = "K"
+}
+
+fun box(): String {
+    val a: A<String, String> = D()
+    return a.o + a.k
+}