libusb: ptest: don't skip debug output and fix failures processing

libusb: ptest: don't skip debug output and fix failures processing

Current run-ptest script prints nothing, when stress tests fail.
Fix it in new implementation, discarding external dependency on sed.
Also leave in place all stress output, just add standard ptest result.

Fixes: 3f0106bf2e41 ("libusb: Add ptest")
(From OE-Core rev: cd05029c78dea48c20f9acb2c5fee56b19193f22)

Signed-off-by: Maksym Kokhan <maksym.kokhan@globallogic.com>
Reviewed-by: Andrii Bordunov <andrii.bordunov@globallogic.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>