Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/controlStructures/kt910.kt

@@ -0,0 +1,23 @@
+import java.util.HashSet
+
+fun foo() : Int =
+    try {
+        2
+    }
+    finally {
+        "s"
+    }
+
+fun bar(set : MutableSet<Int>) : Set<Int> =
+    try {
+        set
+    }
+    finally {
+        set.add(42)
+    }
+
+fun box() : String {
+    if (foo() != 2) return "fail 1"
+    val s = bar(HashSet<Int>())
+    return if (s.contains(42)) "OK" else "fail 2"
+}