tests: git rid of all the tool=tool assignents

Our use of
  ltl2tgba=ltl2tgba
  autfilt=autfilt
  ...
all over the test cases comes from the time where those tools were not
in PATH and we actually had something like
  ltl2tgba=../../bin/ltl2tgba
  autfilt=../../bin/autfilt

But today that is useless, and we prefer to write ltl2tgba rather than
$ltl2tgba, so let's remove this old cruft.

* tests/core/basimul.test, tests/core/det.test, tests/core/lbt.test,
tests/core/lenient.test, tests/core/ltl2dstar.test,
tests/core/ltl2dstar2.test, tests/core/ltl2dstar3.test,
tests/core/ltl2dstar4.test, tests/core/ltlcross2.test,
tests/core/ltlcross3.test, tests/core/ltlcross4.test,
tests/core/ltlcrossce2.test, tests/core/ltldo.test,
tests/core/ltlfilt.test, tests/core/optba.test,
tests/core/prodor.test, tests/core/rand.test,
tests/core/randomize.test, tests/core/remfin.test,
tests/core/satmin.test, tests/core/sbacc.test,
tests/core/strength.test, tests/core/stutter-ltl.test,
tests/core/stutter-tgba.test, tests/core/unabbrevwm.test,
tests/core/unambig.test: Get rid of all tool=tool assignments.

tests/core/ltlcrossce2.test

@@ -1,6 +1,6 @@
 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2015, 2016 Laboratoire de Recherche et
+# Copyright (C) 2015-2017 Laboratoire de Recherche et
 # Développement de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -21,8 +21,6 @@
 . ./defs
 set -e
 
-ltl2tgba=ltl2tgba
-
 cat >fake <<\EOF
 #!/bin/sh
 case $1 in
@@ -63,7 +61,7 @@ EOF
 chmod +x fake
 
 run 1 ltlcross -f 'G(a <-> Fb) U c' \
-  "$ltl2tgba --lbtt %f >%T" "./fake %l >%T" 2> errors
+  "ltl2tgba --lbtt %f >%T" "./fake %l >%T" 2> errors
 cat errors
 grep 'error: P0\*N1 is nonempty' errors
 grep 'error: P1\*N1 is nonempty' errors