use group's basename on test descriptors

In cases where test_groups contains forward slashes, i.e. absolute
paths, only the basename directory should be considered when creating
the test description, otherwise file creation fails as seen below:

    $ test_groups=/home/lsandov1/repos/TF-A/tf-a-ci-scripts/group/tf-l1-build-fvp workspace=$PWD/ws script/gen_test_desc.py
    Traceback (most recent call last):
      File "script/gen_test_desc.py", line 127, in <module>
        process_item(item)
      File "script/gen_test_desc.py", line 90, in process_item
        gen_desc(item, a_file)
      File "script/gen_test_desc.py", line 64, in gen_desc
        with open(desc, "wt") as fd:
    FileNotFoundError: [Errno 2] No such file or directory: '/home/lsandov1/repos/TF-A/tf-a-ci-scripts/ws/000%/home/lsandov1/repos/TF-A/tf-a-ci-scripts/group/tf-l1-build-fvp%fvp-rddaniel-tbb,nil,nil,nil:nil.test'

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: I781e17f2c3ae672669ecdf2252983891e4e8e4af
1 file changed