[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/KotlinTest.sh

@@ -40,7 +40,7 @@ kotlinc $all_kt_files -classpath $targetdir -include-runtime -d $targetdir
 # Make jar
 jar cvf ${testdir}/kotlin_test.jar -C $targetdir . > /dev/null
 # Run test
-kotlin -cp ${testdir}/kotlin_test.jar KotlinTest
+kotlin -J"-ea" -cp ${testdir}/kotlin_test.jar KotlinTest
 # clean up
 rm -rf $targetdir
 rm ${testdir}/kotlin_test.jar