fix typo _tap_get_todo -> tap_get_todo

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
rename test count accessors -> get_testcount_XXX

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
keep track of the number of tests planned

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
add tap_finish

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
t/Makefile: add gcov target

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
document underlying functions

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>