unittests: test_kdoc_parser: add command line arg to read a YAML file

unittests: test_kdoc_parser: add command line arg to read a YAML file

The test_kdoc_parser.py already supports loading dynamic tests
when running unit tests.

Add support to read from a different file. This is useful for:
- regression tests before/afer some changes;
- preparing new unit tests;
- test a different yaml before adding its contents at
  tools/unittests/kdoc-test.yaml.

It should be noticed that passing an argument to a unit test
is not too trivial, as unittest core will load itself the
runner with a separate environment. The best (only?) way to
do it is by setting the system environment. This way, when
the class is called by the unit test loader, it can pick
the var from the environment without relying on a global
variable.

The unittest_helper has already provision for it, so let's
use its support.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <1d1a56de012c43756f9ca87aa9bf6c285674f113.1774256269.git.mchehab+huawei@kernel.org>