tests: avoid spurious failures with non-flex 'lex' programs and C++

tests: avoid spurious failures with non-flex 'lex' programs and C++

This change fixes automake bug#11185.

The commit v1.11-2058-g6f4b08d of 06-03-2012, "tests: explicitly
state that our lexers do not require unistd.h" has broken the tests
'lex-clean-cxx.test' and 'lex-depend-cxx.test' on Solaris, where lex
is not flex and does not understand the "%option never-interactive"
directive.  Remove the use of this directive, resorting to defining
a dummy 'isatty()' function instead to keep the flex-generated tests
able to compile also on MinGW/MSYS.

* tests/lex-clean-cxx.test (parsefoo.lxx): Define a dummy 'isatty()'
function.
* tests/lex-depend-cxx.test (joe.ll): Likewise.
* tests/README: Adjust.  Fix an unrelated typo since we are at it.

Co-authored-by: Peter Rosin <peda@lysator.liu.se>
Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>