change test argument signature to void

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
enable -Wextra for all tests

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

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
t/Makefile: avoid modifying PATH to run tests

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