Redirect install output of make check into a log file

dbf2ec1a changed make check so that the installation logs get directed
to stdout and stderr. Per discussion on -hackers, this patch restores
saving it to a file. It is now saved in /tmp_install/log, which is
created once per invocation of any make target doing regression tests.

Along the way, add a missing /log/ entry to test_ddl_deparse's
.gitignore.

Michael Paquier.