Bison <3.6 used to complain about "$undefined", while Bison >=3.6 now
write "invalid token".
* tests/core/parseaut.test, tests/core/parseerr.test,
tests/core/sugar.test: Adjust expected diagnostics to match Bison pre
and post 3.6.
* tests/core/readltl.cc: Process many formulas from a
file instead of one arg at a time.
* tests/core/parse.test, tests/core/parseerr.test, tests/core/utf8.test:
Adjust to supply a file as input.