ptest-cargo.bbclass: fix condition to detect test executable

before that, a test executable at None was picked.
Moreover, use universal_newlines to subprocess call to avoid
being polluted by fancy carriage return characters.

Signed-off-by: Frederic Martinsons <frederic.martinsons@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>