Tests: tap.sh spams tests' output when no plan is set

Some tests are implemented in C (using tap.h) or in Python
and don't use tap.sh's facilities. However, it is sourced
by utils.sh and prints an error message during its clean-up
because a plan was never set.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>