Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/functions/kt2739.kt

@@ -0,0 +1,10 @@
+// KT-2739 Error type inferred for hashSet(Pair, Pair, Pair)
+
+fun foo<T>(vararg ts: T): T? = null
+
+class Pair<A>(a: A)
+
+fun box(): String {
+    val v = foo(Pair(1))
+    return if (v == null) "OK" else "fail"
+}