tests: run-tests.sh: ensure non-zero exit when $failed != 0

tests: run-tests.sh: ensure non-zero exit when $failed != 0

POSIX [1] does not specify the behavior of `exit' with arguments
outside the 0-255 range, but what generally (bash, dash, zsh, OpenBSD
ksh, busybox) seems to happen is the shell exiting with status & 255
[2], which results in zero exit for certain non-zero arguments.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#exit
[2] https://git.savannah.gnu.org/cgit/bash.git/tree/builtins/common.c#n579

Fixes: 0c6592420586 ("tests: fix return codes")
Signed-off-by: Štěpán Němec <snemec@redhat.com>
Signed-off-by: Phil Sutter <phil@nwl.cc>