[Kotlin] Fix Access to union of vector element (#5994)

Kotlin code generation was producing wrong logic for accessors
of vector of union elements. This was shadowed by the fact[1] that asserts
in Kotlin are silently ignored unless the flag "-ea" is passed to the JVM.

The tests are also updated to enable asserts.

1 - https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/assert.html

tests/union_vector/Movie.kt

@@ -58,7 +58,7 @@ class Movie : Table() {
     fun characters(obj: Table, j: Int) : Table? {
         val o = __offset(10)
         return if (o != 0) {
-            __union(obj, __vector(o) + j * 4 - bb_pos)
+            __union(obj, __vector(o) + j * 4)
         } else {
             null
         }