tests: factor out expensive "pairs" code of misc/stty

tests: factor out expensive "pairs" code of misc/stty

* tests/Makefile.am (TESTS): Add misc/stty-pairs.
* tests/init.cfg (stty_reversible_init_): New function.
(stty_reversible_query_): New function.
* tests/misc/stty: Factor out expensive "pairs" code into new test.
Use new stty_reversible_* functions instead of evaluating static
REV_* variables.
* tests/misc/stty-pairs: Add new test.  Code added from misc/stty.
Mark this as an expensive test.  Skip 'parenb' and 'cread' options,
as these tests are known to fail.  Like in misc/stty, also use
the new stty_reversible_* functions.