[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

src/idl_gen_kotlin.cpp

@@ -942,7 +942,7 @@ class KotlinGenerator : public BaseGenerator {
                   break;
                 }
                 case BASE_TYPE_UNION:
-                  found = "{{bbgetter}}(obj, {{index}} - bb_pos){{ucast}}";
+                  found = "{{bbgetter}}(obj, {{index}}){{ucast}}";
                   break;
                 default: found = "{{bbgetter}}({{index}}){{ucast}}";
               }