tests: explicitly state that our lexers do not require unistd.h

tests: explicitly state that our lexers do not require unistd.h

By default, GNU flex generates code that is dependent on unistd.h,
which is not portable to non-ANSI systems causing spurious test
failures.

* tests/cond35.test (tscan.l): Define YY_NO_UNISTD_H.
* tests/lex-clean.test (lexer.l): Likewise.
* tests/lex-depend.test (joe.l): Likewise.
* tests/lex-lib-external.test (foo.l): Likewise.
* tests/lex-lib.test (foo.l): Likewise.
* tests/lex-libobj.test (foo.l): Likewise.
* tests/lex-line.test (zardoz.l): Likewise.
* tests/lex-nodist.test (lexer.l): Likewise.
* tests/lex-noyywrap.test (foo.l): Likewise.
* tests/lex-pr204.test (lexer.l): Likewise.
* tests/lex-subobj-nodep.test (subl/sl.l): Likewise.
* tests/lex3.test (foo.l): Likewise.
* tests/lex5.test (foo/foo.l): Likewise.
* tests/lexvpath.test (lexer.l): Likewise.
* tests/silent-lex.test (foo.l): Likewise.
* tests/silent-many-gcc.test (foo5.l): Likewise.
* tests/silent-many-generic.test (foo5.l): Likewise.
* tests/lex-clean-cxx.test (parsefoo.lxx): Likewise.  Also specify
the never-interactive option to prevent usage of the non-ANSI
function isatty(3) (from unistd.h).
* tests/lex-depend-cxx.test (joe.ll): Likewise.
* tests/README: Describe these quirks.