lttng-tools: ensure that ptest errors are not suppressed

lttng-tools: ensure that ptest errors are not suppressed

At some recent point lttng-tools ptests have quietly regressed,
and most of them aren't executed anymore. Errors are printed
on the console, but aren't reported as exit code from run-ptest.

The reason is that exitcode was set to what sed returned, not make.
The original reason for piping through sed was to unify /tmp/tmp.xxxx outputs
for easier results comparison, but (after fixing the tests) I don't
see such lines anymore, and in any case ensuring such regressions
are caught is more important.

With this fix, run-ptest and testimage starts to fail as it should.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>