nixos/openldap: check configuration with slaptest

slapd does only print the error and not the line number.
Sometimes it is not even clear that it fails to start
due to an incorrect configuration file.

Example output of slaptest:

5e1b2179 /nix/store/gbn2v319d4qgw851sg41mcmjm5dpn39i-slapd.conf: line 134 objectClass: Missing closing parenthesis before end of input
ObjectClassDescription = "(" whsp
  numericoid whsp                 ; ObjectClass identifier
  [ "NAME" qdescrs ]
  [ "DESC" qdstring ]
  [ "OBSOLETE" whsp ]
  [ "SUP" oids ]                ; Superior ObjectClasses
  [ ( "ABSTRACT" / "STRUCTURAL" / "AUXILIARY" ) whsp ]
                                  ; default structural
  [ "MUST" oids ]               ; AttributeTypes
  [ "MAY" oids ]                ; AttributeTypes
  whsp ")"
slaptest: bad configuration file!