Fix port test that assumed string ports use the `error' conversion strategy.

Fix port test that assumed string ports use the `error' conversion strategy.

This is a followup to 9f6e3f5a997f484548bd03e7e7573c38a95c8d09 ("Have
string ports honor `%default-port-conversion-strategy'.").

* test-suite/tests/r6rs-ports.test ("7.2.11 Binary
  Output")["put-bytevector with wrong-encoding string port"]: Set
  %DEFAULT-PORT-CONVERSION-STRATEGY to 'error.  Return #f when no
  exception is raised.
  ("8.2.6  Input and output ports")["transcoded-port [error handling
  mode = raise]"]: Return #f when no exception is raised.