selftests: ktap_helpers: Add a helper to finish the test

Similar to the C counterpart, keep track of the number of test cases in
the test plan and add a helper function to be called at the end of the
test to print the results and exit with the corresponding exit code.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>