test suites

I'm curious as to what strategies people use to create test suites for
their projects....

I'm working on Yet Another Shop Toaster Oven (in addition to the
standard computer-controlled solder reflow temperature profile, I want
to be able to do things like powder-coat small parts in mine), so I've
got more devices to deal with than most past projects:  the
thermocouple, the zero-crossing detector, the lcd, the timer, the
keypad....

I'd like to get my code working in bite-size chunks:  first get a test
pattern up on an lcd, then interface the keypad.....  the thing is,
when I discover a bug in a later test (say I discover something was
wrong with my lcd driver when I'm working on the keypad), I want to
make sure my bugfix gets propagated back properly.  OK, at some level
the right answer is "make sure I do it", but I've been programming
long enough to have nearly infinite confidence in my ability to make
stupid mistakes.

I started to work on a scheme to break up the code in to various
include files for bit settings, constants, init code, driver code...
and quickly realized that I was creating something that was massively
unwieldy, and also (since I basically had to set things like the
tristate registers with bit-by-but sets and clears) really
inefficient.

So now we come back to my question:  what do you guys do?  Lots of
include files?  Just one big chunk of source code (OK, it sort of
can't be longer than 2000 lines so it isn't that big)?  Some sort of
m4-hell?