"testbenches" are helpful wrappers which exersize a piece of code. they should
live in ./tb/, end in "_tb.v" or "_tb.vhd", and have a signal file (ending in
"_tb.wcfg") associated with the same name. update the 'tbfiles' list in
Makefile. invoke the isim GUI with "make isim/name_tb" (for tb/name_tb.v). if
you make a source code change and don't want to reload the whole GUI, you can
do "make resim/name_tb" and then click the "Re-load" button.

testbenches which can act as headless unit tests (reporting pass/fail by
printing to stdout) should be added to the 'alltests' list in Makefile. run the
test with 'make test/name_tb'; run all tests with 'make tests'.