gRPC channel stays open after test

[fedinskiy]

Method grpcChannel[1] creates managed channel but doesn't close it afterwards and doesn't expose shutdown method of that channel. Because of that, if we run several test classes in a row, we get warning, that "previous channel was garbage collected without being shut down!".

[1] https://github.com/quarkus-qe/quarkus-test-framework/blob/main/quarkus-test-service-grpc/src/main/java/io/quarkus/test/bootstrap/GrpcService.java#L9