tests: initialize fail=0 to avoid "unary operator expected" errors

tests: initialize fail=0 to avoid "unary operator expected" errors

With an uninitialized variable 'fail', the unquoted use like
  test $fail = 1
lead to the shell error
  "unary operator expected".

The uninitialized 'fail' variable was a side effect of
  https://git.sv.gnu.org/cgit/gnulib.git/commit/?id=e91c0d4f9
which was pulled into coreutils-v8.26 with
  https://git.sv.gnu.org/cgit/coreutils.git/commit/?id=ef9650170
Coreutils test code relied and relies on 'fail' to be initialized,
so initialize that variable here.

* tests/local.mk (TESTS_ENVIRONMENT): Initialize fail=0.