minor: kunit: tool: fix unit test so it can run from non-root dir

Also take this time to rename get_absolute_path() to test_data_path().

1. the name is currently a lie. It gives relative paths, e.g. if I run
from the same dir as the test file, it gives './test_data/<file>'

See https://docs.python.org/3/reference/import.html#__file__, which
doesn't stipulate that implementations provide absolute paths.

2. it's only used for generating paths to tools/testing/kunit/test_data/
So we can tersen things by making it less general.

Cache the absolute path to the test data files per suggestion from  [1].
Using relative paths, the tests break because of this code in kunit.py
  if get_kernel_root_path():
          os.chdir(get_kernel_root_path())

[1] https://lore.kernel.org/linux-kselftest/CABVgOSnH0gz7z5JhRCGyG1wg0zDDBTLoSUCoB-gWMeXLgVTo2w@mail.gmail.com/

Fixes: 5578d008

 ("kunit: tool: fix running kunit_tool from outside kernel tree")
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Acked-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>