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/kt3203_2.kt

@@ -0,0 +1,20 @@
+fun check1() {
+    val result = if (true) {
+        if (true) 1 else 2
+    }
+    else 3
+    if (result != 1) throw AssertionError("result: $result")
+}
+
+fun check2() {
+    val result = if (true)
+        if (true) 1 else 2
+    else 3
+    if (result != 1) throw AssertionError("result: $result")
+}
+
+fun box(): String {
+    check1()
+    check2()
+    return "OK"
+}