ssh/channel-type.c (print_channel): Use `SCM_OPPORTP' predicate
tests/client-server.scm (spawn-server-thread): Catch all exceptions

* tests/client-server.scm (spawn-server-thread): Catch all exceptions
  to fix the "Error while printing of exception" during the tests.
* tests/server-client.scm (spawn-client-thread): Likewise.