In addition to the main subunit file from zuul_work_dir,
collect the subunit files from the elements (directories)
of the zuul_additional_subunit_dirs list.
The default behavior is unchanged.
While the documentation of this role states that zuul_work_dir
contains an absolute path, this is not always true.
So make sure to not make any assumption about zuul_work_dir
in order to not fail spectacularly as it happened with the
previous patch[0].
Add also some tests for the role: both the basic case
and with an additional test directory.
[0] https://review.opendev.org/673885
Change-Id: Iabf2e0cf6d86e36a174778367186bbd39a65c3dd
ab8f9fc403