tests: Keep .log files in objdir.

* tests/gpgscm/tests.scm (open-log-file): Keep the log file in objdir.

Before the change, it is at ephemeral temp directory which is removed.
This is not useful at all. Possibly, it was done before the introduce
of ephemeral temp directory for each test and not changed.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>