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/kt3273.kt

@@ -0,0 +1,21 @@
+fun printlnMock(a: Any) {}
+
+public fun testCoalesce() {
+    val value: String = when {
+        true -> {
+            if (true) {
+                "foo"
+            } else {
+                "bar"
+            }
+        }
+        else -> "Hello world"
+    }
+
+    printlnMock(value.length)
+}
+
+fun box(): String {
+    testCoalesce()
+    return "OK"
+}