ptest-cargo.bbclass: add the possibility to define test arguments

ptest-cargo.bbclass: add the possibility to define test arguments

It can be useful for user of this class for giving common rust test
arguments (like --shuffle, --format or --test-threads)

To add arguments to test binary , simply define RUST_TEST_ARGS
in your recipe that inherit ptest-cargo, for example:

RUST_TEST_ARGS = "--shuffle"

Signed-off-by: Frederic Martinsons <frederic.martinsons@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>