Add a first test applying Scala 3 Union types #1238

Job	Run time
Compile (testkit/Test/compile actor-tests/Test/compile)	3m 44s
Compile (actor-testkit-typed/Test/compile actor-typed-tests/Test/compile)	4m 5s
Compile (bench-jmh/Test/compile)	6m 14s
Compile (cluster/Test/compile cluster-tools/Test/compile cluster-typed/Test/compile distributed-d...	7m 44s
Compile (discovery/Test/compile coordination/Test/compile)	4m 39s
Compile (persistence/Test/compile persistence-shared/Test/compile persistence-query/Test/compile ...	7m 44s
Compile (pki/Test/compile slf4j/Test/compile)	3m 23s
Compile (serialization-jackson/Test/compile)	4m 37s
Compile (stream/Test/compile stream-testkit/Test/compile stream-tests/Test/compile stream-typed/T...	6m 6s
Compile (stream-tests-tck/Test/compile)	3m 41s
Compile (remote/Test/compile remote-tests/Test/compile protobuf-v3/Test/compile)	5m 9s
	57m 6s

Provide feedback