Try to avoid `guile-test' failures when it can't display the name of a test.

Try to avoid `guile-test' failures when it can't display the name of a test.

This can happen, e.g., when printing "UNRESOLVED: regexp.test: TEST-NAME",
where TEST-NAME contains characters that cannot be converted to the
encoding of the current output or error port.

* test-suite/guile-test (main): Set the conversion strategy for the
  output and error ports to `escape'.

* test-suite/tests/regexp.test (current-output-port): Likewise.