This makes it a bit cleaner when dealing with optional tests as we
use the LIST(APPEND...) command everywhere instead of sometimes SET
and sometimes LIST.

Since we have random files we want to read & check, turn the two
read programs into general tools.  Then we have shell scripts to
run against the right inputs.

Most of the changes here are just shuffling variable names so we
can add to check_PROGRAMS independently of TESTS.

This matches what we've done with cmake files, and it makes managing
the subdirs much easier.

This makes the test code a lot easier to manage rather than copying &
pasting the same boiler plate multiple times.

Also take the opportunity to add a common prefix to each test name so
we don't get collisions between subdirs if a test happens to use the
same exact name.

This makes it easier to keep track of things.  The single toplevel list
can make it hard to notice when something is stale.