src/poky:ptest-cargo.bbclass: create class

This new class offers the possibility to build rust unit tests
(and integration tests) and find them correctly.
Due to non deterministic names of generated binaries, a custom
parsing of build result must be performed.
See rust-lang/cargo#1924

All rust projects will generate a test binary with "cargo build --tests"
command, even if there are no test defined in source code.
The binary will just output that it ran 0 tests.

(From OE-Core rev: dad9bad239d757ae0b159fe5f1276b6856547b4c)

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>