tests: replace non-portable use of sed by $PERL

This fixes one failure reported in #428 by Étienne Renault.

* tests/core/lbt.test: Here.
* tests/run.in: Export PERL.

tests/core/lbt.test

 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2013, 2016, 2017, 2019 Laboratoire de Recherche et
+# Copyright (C) 2013, 2016, 2017, 2019, 2020 Laboratoire de Recherche et
 # Développement de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -99,7 +99,7 @@ test `wc -l < formulas.4` -eq 168
 
 # Add some carriage returns to simulate MS-DOS files and
 # make sure our parser does not mind.
-sed 's/$/\r/' formulas.2 > formulas.2ms
+$PERL -pe 's/$/\r/' formulas.2 > formulas.2ms
 # The --csv-escape option is now obsolete and replaced by double
 # quotes in the format string.  So eventually the first two lines
 # should disappear.

tests/run.in

@@ -61,6 +61,9 @@ export top_builddir
 top_srcdir='@abs_top_srcdir@'
 export top_srcdir
 
+PERL='@PERL@'
+export PERL
+
 # Reset those variables, as they may affect the output of DOT.
 SPOT_DOTEXTRA=
 export SPOT_DOTEXTRA
@@ -108,7 +111,7 @@ case $1 in
   *.test)
     exec sh -x "$@";;
   *.pl)
-    exec @PERL@ "$@";;
+    exec $PERL "$@";;
   *python*|*jupyter*)
     PYTHONPATH=$pypath DYLD_LIBRARY_PATH=$modpath \
     exec $PREFIXCMD "$@";;