tests,w32: Fix gpgsm signature verification test.

* tests/gpgsm/verify.scm: Use 'call-with-binary-output-file' to avoid
automatic line-ending conversion.

Previously, Windows' automatic line-ending conversion turned '\n' into
'\r\n' when writing the sample to a file, making the signature
invalid.

• Signed-off-by: Justus Winter <justus@g10code.com>