Allow slapd path configuration in t_kdb.py

The upstream OpenLDAP installs slapd in libexec, which is not
typically in the path.  Also, copying the binary can sometimes cause
it to fail; for instance, in the OpenCSW package,
/opt/csw/libexec/slapd is a script which chooses a binary based on the
system architecture and the path to the script.  Allow the test runner
to set the SLAPD environment variable to specify the slapd location
and avoid the copy.

(cherry picked from commit 9290e52b7f206dcbfd9dd5d873cf9379cb9de760)

https://github.com/krb5/krb5/commit/4bda951811e3a7b17a3124ac79378797a8a425b8
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 4bda951811e3a7b17a3124ac79378797a8a425b8
Branch: krb5-1.15
 src/tests/t_kdb.py |   17 ++++++++++-------
 1 files changed, 10 insertions(+), 7 deletions(-)