Move ir box test under "box/ir"

compiler/testData/codegen/box/ir/fileClassInitializers.kt

@@ -0,0 +1,15 @@
+fun <T> assertEquals(actual: T, expected: T) {
+    if (actual != expected) {
+        throw java.lang.AssertionError("Assertion failed: $actual != $expected")
+    }
+}
+
+val x = 1
+val y = x + 1
+
+fun box(): String {
+    assertEquals(x, 1)
+    assertEquals(y, 2)
+
+    return "OK"
+}