lib/tst_run_cmd: Make it possible to detect ENOENT

Previously the code did _exit(-1) on any execvp() failure and detecting
ENOENT and possibly exit the test with TCONF wasn't possible.

Now we do _exit(255) on ENOENT and _exit(254) in the rest of the cases.

+ Update test-writing-guidelines

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
Suggested-by: Jan Stancek <jstancek@redhat.com>
2 files changed