tests: Keep .log files in objdir.

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

Forward port from 2.2 branch of:
1c88104a3f00f7ca3790fbaab8f67b2b68cd6e18

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>