tests: don't define YACC and LEX in the Makefiles

tests: don't define YACC and LEX in the Makefiles

We don't want YACC and LEX defined as make variables, otherwise
the values determined at configure time will override those from
the environment, even in the make-spawned testcases.  For example,
before this change, with the following usage:
  $ ./configure YACC=yacc
  $ export YACC='bison -y'
  $ make check
the testsuite would have ended up, very counterintuitively, with
YACC defined to 'yacc' in the testcases' environment.

* configure.ac: Call `AM_SUBST_NOTMAKE' on YACC and LEX.