src/poky:bitbake: bb/tests: use subtests when looping in a test case

Marking the test iterations as subtests means that when one fails, it
can identify clearly which iteration has failed.

(Bitbake rev: 52c55e681332d7cdbe06f3c9d9c8d77cb0cb93f6)

Signed-off-by: Ross Burton <ross.burtonarm.com>
Signed-off-by: Richard Purdie <richard.purdielinuxfoundation.org>

Author: rossburton